Hi,
You can do hub & spoke MPLS VPN without the VTI's VTI's is one option where you can use it as Half Duplex VRF for the cases that two spokes connected to same PE when spoke's are on same VRF.
But if you can configure each spoke to a different VRF this problem can be overcome and you have to play with route-targets.
Main point to taken on Hub & Spoke is any spoke to spoke communication should happen via Hub-CE router.
It is possible to configure hub & spoke MPLS VPN with subinterfaces.
Rgds,
Harin