That is not possible.
The VPLS tunnel itself cannot be encrypted, since in RouterOS, MPLS and VPLS traffic doesnt pass the L3 routing logic, where IPSec processes are placed.
MPLS handlers are located before IP Routing handlers, so the VPLS traffic never even gets to the routing engine (and IPSec engine).
And since you are just briding VLANs inside of the VPLS tunnel, no way to encrypt/decrypt those either, since L2 processing is also done before the IPSec engine.
What you could do is build an EoIP tunnel, and enrcypt the EoIP tunnel. Since EoIP is GRE, you can use IPSec in transport mode to encrypt it.
Here is the packet flow diagram, where you can see why you cant do any IPSec for VLANs inside VPLS:http://forum.mikrotik.com/viewtopic.php?f=2&t=72736