Almost every historian of the late Roman Empire repeats as fact that Stilicho was the son of a Vandal father and Roman mother. However, it seems like the only source for this is from Orosius 7.38:
Count Stilicho, who was sprung from the Vandals...
Orosius makes a lot of clearly false claims about Stilicho, saying that he wanted to persecute the Christians and giving him no credit for his defeat of Radagaisus. Meanwhile, Stilicho married into the family of the emperor, which seems unlikely for a "barbarian", especially one not of particularly notable barbarian birth. Are there other sources for Stilicho's heritage and if not, how much trust should we put into Orosius' claim?
As far as I know there aren't really any other sources for his heritage other than Orosius. I think that the claim of him being the son of a 'Vandal' and of not notable birth is pretty accurate since if there was something notable in his pedigree we could have expected Claudian to make a mention of it. For example in his "On the Consulship of Stilicho" Claudian simply passes over his father.
Also with regard to his marriage into the imperial family I think it's important to consider that many of the leading men during the late antique period (Especially the 3rd and 4th centuries) rose from low ranking families to incredibly high positions within the empire by use of the army (Take Diocletian for example).This is just my thoughts on it and I'm sure someone else might have more knowledge on this topic.