AFAIK, this is a much under-researched topic, but then also one the Vietnamese governments were never particularly keen to let be researched, and - seemingly - one even the US researchers not being keen to research. A few (perhaps 2) related articles have been published in Russia over the time, foremost in the late 1980s and early 1990s (times of 'Perestroyka'). Few additional narratives appeared on Russian websites specialised in collecting veteran recollections. However, even there only 'episodes' relating to a few officers have been described. Over the time, I think I have heard (not read, though) about 2-3 books written by Russian (or 'other ex-Soviet') authors, and published in English, about various of Soviet 'interventions' abroad. However, from what I've heard about works in question, all are rather general by their nature, and not going into 'such details' like participation of Soviet pilots in specific combat operations.

From articles in question (sorry, can't find any of them right now; I wasn't very careful with sorting out my files at the time, but related write-ups, posted back in 2004, can still be found on ACIG.info forum; this requires a registration to read) it transpired that up to 30.000 Soviet military advisors have served in North Vietnam between 1960 and 1975. Most of them were organized into 'special groups' and served as advisors, i.e. trained various units and branches of the North Vietnamese military.

At least during the 1960s (i.e. until the 'bombing halt' of 1968) this role often resulted with Soviet advisors for Vietnamese air defence units taking part in combat operations too. This was so because at least one Soviet advisor was deployed with de-facto every single North Vietnamese battalion equipped with surface-to-air missiles (SAMs; in turn, one battalion was equivalent of what is colloquially known as a 'SAM-site' in the West). A few higher-ranking officers were responsible for collecting and studying captured US-military equipment (much of which was then sent to the USSR for further studies).

One particular example I do recall to have read about was a 'special group' composed of about 20 Soviet instructor pilots, deployed in North Vietnam to train local pilots to fly MiGs. On average, they served there for up to 12 months, whereupon they would be replaced by another group of 20 pilots. Supposedly, this practice was continued until after 1975.

The pilots in question did not take part in combat operations: they were flying training sorties on two-seaters (like MiG-15UTI and MiG-21U) with North Vietnamese students in front cockpits. However, a few times they have been taken by surprise by US aircraft and forced to defend themselves. One case I recall quite well happened in May 1968: a Soviet instructor underway with a North Vietnamese student on a MiG-21U (well-known, supersonic jet) was attacked by a pair of US Air Force F-4 Phantom IIs. The MiG was eventually damaged by a single AIM-7 Sparrow air-to-air missile and then crashed on landing, killing the crew (or at least the Soviet advisor).

There are legends that this was something like the 'drop that spilled the barrel' - in so far that afterwards the Soviet instructors should have been ordered to fly combat sorties too, foremost on MiG-21s, and at least for the next six months. Supposedly, this 'phase' ended in late 1968 (the story is that a Russian researcher kind of 'lifted' a memorandum from the archives of the Soviet Ministry of Defence; dated with 11 November 1968, this should have cited something like an order that Soviet pilots were, '...no longer to assist the North Vietnamese air force in training flights when these resulted in engagements of US aircraft').

What's making me suspicious of this version is the following. Firstly, if they did fly any air combats, the Soviets did not claim any kills. This would be rather surprising, considering how much were Soviet advisors posted to various Arab- and African air forces of the 1970s - boasting about performance of MiGs and their armament during the Vietnam War.

Secondly, based on my related write-ups, the Russian sources in question are citing the arrival of 'that' special group of Soviet pilots in 1968, and that this group arrived together with a group of 40 North Korean pilots who were ordered to help North Vietnam.

However, French researcher Albert Grandolini - who is semi-Vietnamese and not only has a unique (and giant) collection of official documentation, but has also travelled all over the former North Vietnam over the years - has found (and photographed) at least half a dozen of graves of North Korean pilots killed in this war. The graves in question were dated with 1966-1967. For example, at least one of North Korean pilots should've been shot down and killed during the famous Operation Bolo (January 1967, when USAF ambushed and shot down 6-7 MiG-21s). Means: North Koreans were in North Vietnam already much earlier.

(Note: after supporting the work of numerous US researchers over the last two decades, like Robert Mikesh, Grandolini has recently started publishing his own books too. So far, nearly all of these are related to 'general warfare', or to the former South Vietnamese air force; i.e. we have to wait for him finding time and opportunity to write something about the North Vietnamese, too.)

Why is this of any importance or relevance? Because much of 'popular history' about Soviet instructors serving abroad is at least vague (most of related accounts are lacking dates, for example), often unreliable, and sometimes entirely wrong (like when specific narratives 'recall' some air base having two runways, while the bases in question only have one runway, and this until this very day). Cases in question are primarily those written by people who might have heard a story or two about somebody serving 'there' (whether in North Vietnam, or whatever other 'Soviet client' country around the globe), but have never been there.

In some cases, it seems the stories about involvement of Soviet pilots were actually based on little else but involvement of Soviet aircraft, i.e. aircraft Soviet instructors assembled and test-flew in North Vietnam, before handing them to the North Vietnamese.

The problem is enhanced by the fact that the known history of the North Vietnamese air force at that war is the 'official' version of this story: this is leaving out any kind of participation of foreigners on the North Vietnamese side (indeed, that version 'deleted' the participation of many North Vietnamese pilots that fell in disfavour with the government in Hanoi over the time).

The North Vietnam's official story is presenting a 'clear and tidy' impression of its air force at that war in regards of its photographs too: it's always showing aircraft wearing perfectly tidy North Vietnamese markings. In comparison, there are clear and dependable indications many of Soviet-delivered aircraft were flown into combat 'as delivered'. For example, in his interview for the Project Red Baron (USAF's study of air combats over North Vietnam, totalling about 2,000 of pages), Roger Locher - a WSO of the USAF F-4D that nearly collided with a MiG-21, shortly before being shot down by a pair of J-6s (Chinese-made MiG-19S'), on 10 May 1972 - clearly recalled the MiG-21 in question wearing the blue serial '53'; i.e. 'Soviet-style bort number' instead of standard North Vietnamese four-digit serial, which was always applied in red.

Overall, I would say the story of the Soviet pilots flying combat sorties during the Vietnam War remains a topic that requires a lots of additional research.