Could a 2900 be used behind a DSL router purely for VPN termination ?
The other DSL router would be set to forward all VPN traffic to the 2900.

Would I be able to use content filtering, port throttling and VLANS on the 2900 LAN ports ?
Any issues using it like this ?

Yes, but the V2900 is a very old model so I don't know if it supports content filtering. Also, the other router would need to forward VPN nicel and not mess it up with NAT.

I know it's an old model..

But I have the change of a very cheap one... so for VPN termination it could be a quick, cheap SECURE ? option..