Is it possible to use bandmin for name based virtual host ? If it's not possible, what would be the best software to use to check bandwidth usage on name based vhost ?

Sorry, I can only answer your first question.

I don't believe bandmin will work for name-based virtual hosts. It uses the kernel interface stats, and there is only one interface for each IP address. All your named-based hosts would appear as one "lump" amount.

On a vhosting, you will need to monitor each service for the usage by each vsite.

For example, add up usage from proftpd, apache & qpopper for ftp, web & mail transfer.

This will not give you the exact number, but will be about as good as you can get considering the fact that it is much harder than monitoring on an IP basis.

I don't know of any tool that does that already (besides control panels including it)