Wait in DMC1 Trish said he lost a brother and a mother at the age of 8....
Have you ever considered that it could mean that Vergil walked out on him AFTER the death of their mother?
After all, it is implied in DMC3 that they haven't seen eachother since they where kids.
Maybe Vergil could have simply abandoned Dante in his quest for power and was never heard of from Dante until the events of DMC3.
If you ask me, that sounds just like something Vergil would do.