II was going to say that I didn't think you could create vlans an an interface that is already plumbed, but it looks like 8.0 does allow tagged and untagged traffic on the same physical interface - but you should be sure that that is, indeed, what you want to do. If not, you'd need to ifconfig down sanvif2, vlan create (include a vlan for the existing network), then ifconfig all three vlans. Otherwise, I think this looks okay. Remember to put your config lines in /etc/rc as well.
What commands did you use? What does ifconfig -a show? To me, this looks like you've created a single vlan (sanvif2-50). The "vlan indices: 50" line is on the stanza describing the physical link. If you had created a 50 and a 100 vlan, I think it would say "vlan indices: 50 100".