There is definitely LNS functionality (but not LAC).
VRF’s i’m personally not used in this instance - but dynamically it doesnt seem possible, but if you only have a few who require it you could set up custom pppoe server interfaces for them that pushes them into VRF (see: http://forum.mikrotik.com/t/how-to-put-dynamic-interfaces-to-specific-vrf/40852/1), this could be a management nightmare though if you are running multiple LNS and/or have many of these custom VRF links.
RouterOS currently supports LNS, including with tunnel encryption. The feature set is quite limited in comparison to say Cisco VPDN+SSS but it will work for most basic scenarios. LAC is currently NOT supported, this would likely need the implementation of something akin to SSS, so would be a major undertaking for Mikrotik.
As for setting the VRF to terminate a connection to with a RADIUS VSA. This is currently NOT supported. Hopefully Mikrotik add this feature at some point.
How would I route particular sessions via a differnt route table? Would it be based on the PPP details? As I’m only able to provide the upstream with one L2TP endpoint.