I created Ethernet subinterfaces with VLAN and isl/dot1q
then i tried Applying VRF (part of MPLS VPN) to it, and it worked
without giving error.
But I believe, VPN isnt supported on VLAN interfaces.. it doesnt make
sense to have L3 tunneling inside L2.
I dont know if this configuration works in real, but it is accpeted by
Yes. It will work perfectly fine. Just 1 thing to remember. Configuring vlan on the subinterface is not the same as configuring vlan on a switchport. A subinterface is still an L3 interface and the vlan tag is just used to distinguish traffic on 1 subinterface from another subinterface. So a subinterface with Vlan encap will work the same as a pure L3 interface. Vlan encap is mandatory for a subinterface. Also you would have to configure ip address on the subinterface to make the vrf useful.
If you configure vlan on a switchport (eg access / trunk) then it is a L2 interface and VRF doesnt make sense on it and is not supported.
Hope this makes things clear.