This is a very tricky question to answer, especially as no records exist to track such lineages and the "racial" composition of Western Europe was mixed up profoundly in the aftermath of Western Rome's decline. This isn't my area but I'll have a go for you.

You are correct that many barbarians served in the Roman army, and land was a part of the deal. This took many forms, such as foederati who were landed and paid in food for a few years until they got their farms up and running (Southern 259), or the laeti, barbarian colonies that were settled but expected to provide regular troops for the army (259-60). The late Roman army was heavily staffed by Franks or even Vandals (Stilicho) in senior positions (260). Conscription also occurred from other landowners, who were expected to provide recruits from large estates (257-8), and among these more 'civilian' populations, self-mutilation to avoid serve was so common as to warrant legislation (258). Some barbarian soldiers in this period had one foot in the door of the army, but were also semi-agrarian in order to compensate for irregular army payments, or served as private soldiers for landowners (Fear 457-8).

During this late Imperial period, the blending of Roman and "barbarian" populations was the natural state of affairs; as an anonymous commentator put it, "The poor Roman imitates the Goth, while the rich Goth imitates the Roman" (Fear 457). Entire groups of ex-soldiers might settle and become known as a distinct group (458). Post-Roman kingdoms continued to be served by ex-Roman troops, who retained their Roman identity as much as they could in some instances, including dressing in a Roman style (Procopius Wars 5.12.17). As Fear put it, "Ironically...after a long period of distrust and dislike, soldier and civilian may have become more united after the end of the Empire than they had been while it existed, and the army emerged as a better defender of Romanitas after the collapse of Rome than it had while it survived.

The "identity" of the groups populating Western Europe after the mid fifth century AD is not a simple matter. Gaul was settled heavily by barbarian-origin Franks and Burgundians, who eventually considered themselves descendants of Romans or even the Trojans, while professional Roman military forces blended back into society when central government failed (Stickler 510). The identity of these post-Roman groups was potentially very fluid, for example Franks might have considered themselves Franks because they had kings who forced that identity on them. New groups of Europeans also emerged from the variety of Celts, Romans and barbarians that populated Western Europe, but under the names of older groups, such as Lombards, Bavarians, Saxons and Visigoths (Bennett 32). Polygymy was common and further complicated matters. The last major barbarian confederation to enter Western Europe was the Lombards in the 560s, who formed a kingdom that is still remembered by what is now Lombardy (37). Around this period the major migrations and intermixing of races and cultures must have stabilised (until the whole Vikings thing), but that doesn't simplify things much does it?

While monarchies and empires rose and fell after this period of rapid intermixing of peoples and cultures, and other peoples such as Vikings and Muslims came and shook things up, no great cataclysm uprooted the population of Western Europe that had settled by the 500s AD in the ruins of what we now call the Western Roman Empire. So while I don't know how I could identify any particular European landed family who originated as descendants of a Roman veteran, and it is profoundly unlikely that any such family group still exists on the same land as they did back then, it is all but certain that large numbers of people in Western Europe, and the rest of the western world, are somehow descendants of one of the many varieties of soldiers that could be called "Roman" veterans.

Sources:

Bennett, Medieval Europe: A Short History, 2011.

Fear, "War and Society", from the Cambridge History of Greek and Roman Warfare, 2008.

Southern, The Roman Army: A Social & Institutional History, 2006.

Stickler, "The Foederati", fro A Companion to the Roman Army, Paul Erdkamp (ed), 2011.

There was a several century gap between the fall of the Western Roman Empire and the rise of Charlemange's empire. Practically all European monarchies can be traced back to Charlemange, but there is no certain connection of Charlemange to antiquity. There was simply nobody keeping records in the interim, and as such there is no known person with a confirmed link to antiquity.