Olof Palme, the Prime Minister of Sweden 1969–76, was one of the many supporters of the Viet Cong, which influenced the Swedish government's involvement in the question.
From the mid-60s onwards, the conflicts in Vietnam (and later, US involvement in Vietnam and elsewhere in the region) was a common talking point of his.
One of the first criticisms that created international interest was at February 21 1968, where he participated in a demonstration (arranged by the United NLF Groups as touched upon later) alongside North Vietnam's ambassador in Moscow Ngyuen Tho Chan. The speech that he gave was a summary of many of his views (an English transcription can be read here).
Another speech given in 1972 comparing the recent US bombings of Hanoi with Treblinka (among other things, the speech with English subtitles can be seen here) created even more commotion, and led to frozen diplomatic relations with the US.
Olof Palme did however not share the political views of the Viet Cong. His main focus was on recognising the horrors of war. As such, I think that it's more fair to see him as a proponent for peace rather than a proponent of North Vietnamese ideology.
If you know Swedish, Here is an article describing Palme's views and actions during the Vietnam war in more detail (with references given that also apply to this post).
Much of the support did not come directly from the Swedish government itself, but instead from popular movements such as DFFG (De förenade FNL-grupperna, "United NLF Groups"). This was a widely successful group, and probably did a lot more than Olof Palme when it comes to seeing Sweden as supporters of the Viet Cong. Their ideological involvement varied (both within the organisation and over time), but peace was their core issue.
There were also other pro-Viet Cong groups, such as KFML(r), which notably during 1973 held a collection with the goal of buying an anti-aircraft gun for the Viet Cong (Lars Gyllenhaal & Lennart Westberg, Svenskar i krig 1945–2015, online at google books).
It should be noted that while the Prime Minister of Sweden and a very large popular movement was in favour of Viet Cong in various ways, the Swedish government did not officially support either side, remaining neutral (but with very close ties to NATO). The United NLF Groups frequently criticised the Swedish government's position on Vietnam.
(^^also, ^^pinging ^^/u/vonadler ^^as ^^he ^^probably ^^knows ^^a ^^lot ^^more ^^about ^^this ^^subject )