Hi:
That question is asked a lot on these forums. I wonder if doing a search would help you.
But to answer your question, one way of spanning a vlan across a L3 cloud is to tunnel through it, such as with L2TP or GRE tunnels. There are also L2 MPLS-based VPNs that you can look into.
Off the cuff, I would say GRE tunnels are the easiest to set up.
HTH