i have ran a slann twice so far and every time against VC no matter what i try to do they get around and slam in the back and kill the tg and run the slann down, i usually move hum behind my battle line but that probably isnt the best idea, any advice would be highly appreciated.
When you field a slann, he always comes with a big red target painted on his forehead (not actually, but you get it). The key to keeping him out of trouble is to give him very good protection. Normally this comes in the form of temple guard, but there are other ways. In a 2000pts match, I always give my slann a 16 strong unit of TG. This goes a long way to protecting him. With this unit, he can be in the thick of it and still be able to cast his spells. From what it sounds like, your trouble seems to be coming from your flanks. If I knew what was getting behind you and what has failed to stop it, I could help you more. I also have a VC army, so I know the ins and outs of their army.
it was some sort of vamp on a flying mount that counts as a regular mount, and he had some flaming lance, he charged me in the flank and attacked the slann, who had his back showing because had no tg behind him.
Sounds like you either have too few Temple Guards, or someone is flanking you big time. So either go with a bigger unit of TG's around your Slann, or try to focus a bit more on defing thier flanks with skinks or perhaps some CoC maybe.
TG seem quite expensive for a 2k pt game, a lone slann would give you more models in the army. its not so hard to protect a slann. give him a divine plaque of protection if you are worried about shooting and spells and keep him in the middle of the army. keep him outside of the line of sight of flyers and surround him with a unit of skink skirmsihers. anything that tryes to get close will get poisoned by skinks while he blases away.