Q:

In MPLS VPN, Which one become first? MPLS Label tag or VPN encapsulation?

In MPLS VPN network, which one is the first thing to do? I mean label tagging is first or VPN encapsulation is first? If you can, please explain about this case.

A:

I think your question is not truly correct.
First of all, as explain here 
"an MPLS VPN is a VPN that is built on top of an MPLS network, usually from a service provider, to deliver connectivity between enterprise office locations. "
The MPLS is the mechanism that directs data from one network node to the next, based on short path labels instead network addresses. This avoid the routing table complexity.
MPLS works in conjunction with IP and its routing protocols.
Based on this, MPLS VPN is a method to harm the power of MPLS to create virtual private networks.
There are three different type of MPLS VPN:

Layer 3 MPLS VPNs
Layer 2 MPLS VPNs
Point-to-point (pseudowire)

So, the protocol on the base of all this environment is the Label Distribution Protocol (LDP), which routers, capable of MPLS, exchange label mapping information.
Label tagging is developped to works MPLS network also without VPN.
Then, MPLS VPNs born to combine the power of MPLS and the Border Gateway Protocol (BGP) routing protocol: MPLS is used to forward packets over the provider’s network backbone, and BGP is used for distributing routes over the backbone.
The common thing (if you analyze a packet captured in a MPLS network) is the MPLS header that is (in a L3 MPLS) always putted between datalink layer and IP layer.

In a L3 MPLS VPNs, for example, the situation is this:

In details, the MPLS header frame format is:

