Any way to reduce bind9 memory footprint?

You can take a look into options like:

• datasize [bytes]
• max-cache-size [bytes]
• tcp-clients [number]
• recursive-clients [number]

These are all global options that may actually cause your server to stop responding once hitting the caps, but will limit the memory used.

For a real world example, with a setting of "max-cache-size 1M" my BIND install is using just about 30-35MB of memory (uptime 2 weeks, authoritative for 9 zones, light traffic).

Maybe not answer you are looking for, but why not go with a hosted DNS? I guess I am not a true geek sysadmin but I offloaded admin stuff that was not so useful for us (email, dns) to dedicated companies instead of wasting my time on it. We use www.dnsmadeeasy.com and they have been great. The prices are really cheap for a few domains. It will be cheaper than your VPS, that's for sure.

I'd take a look at TinyDNS also, its another more popular BIND replacement