If you create a default gateway pool, then auto lasthop should take care of your first concern. I think as long as you only enable the SNAT and the VLAN X virtual server on VLAN X (instead of the default of All VLANS) then the traffic from the SNAT to VLAN Y will go out the appropriate default gateway. But without being able to test your setup I'm not 100% sure that what you describe won't happen, because without the forwarding virtual server you can't explicitly point outgoing traffic to the firewall interface.
Denny