Vostroyans have a preference toward Plasma and bolt weaponry (specifically the Heavy Bolter) for squads (I'll have to try and find the source for this, but it was in one of the previous guard codexes I believe). They do also utilize flamers a lot as they are generally a force that specializes in urban combat. I personally have a trio of plasma guns, a trio of melta guns (with a 4th meltagun conversion waiting to be painted), a quartet of flamers, and a quartet of sniper rifles for special weapons. I also have a trio of lascannons, a heavy bolter, and a mortar as well for heavy weapons. The point is, run whatever weapons you like. They're all useful in their own way. I personally prefer plasma, but having a melta or flamer team can be very very helpful as well.
In addition, I tend to find guard heavy weapon teams are not all that useful, and often get targeted early and often with
ST:6 or higher weaponry, which results in them dying before they do much of anything. Your mileage may vary though.
I own a fairly sizable force of Vostroyans and I typically run nothing but Veteran Squads to represent the carapace armor with the grenadiers doctrine. I also usually spring for Krak grenades instead of Forward Sentries as nine times out of ten I have them mounted in Chimera transports.
If you choose to run a 'regular' platoon, I suggest doing what others have suggested and converting up using third party bits, OR, go in a completely different direction and have a combined regiment of both Cadian (for the infantry platoon) line troops, and Vostroyan as your Veteran squads.
This happens a lot in the lore, so representing it on the tabletop could be fun.
Hopefully that helps. If you need any painting tips, feel free to shoot me a
PM. I've got a few of my models and tanks for my Vostroyans in my Gallery if you'd like to take a look. Nothing spectacular but they're painted to a pretty good tabletop standard.
Take it easy.
-Red__Thirst-