VM's of different segments in same ESX host?

Hello Friends,

I am planning to setup ESX 5.1i cluster hosts of say 3 hosts. In this cluster I have to do setup VM's which are part of different internal segments.i.e there are 2 segments which are actually separated by internal firewall in current physical setups. Now when I do P toV and club these VM's in one single ESX host, is there any issues or concerns with respect to security. I.e As I mentioned in current physical setup ( without virtualization) these two segments are between firewall.

Is VCNS is must to do this kind of setup.As that will cost a lot,is there any other way around?

Thanks

siri