PPSSPP to Vita Multiplayer?

[AdamN]

Depends on the game whether they use ports under 1024 or not, for games that use port under 1024 it's not possible when using the same network/LAN, due to Android restriction not allowing to bind/listen on ports under 1024.

Might be possible over the internet (if your ISP allow port-forwarding on ports under 1024) by enabling "UPnP use original port" setting on PPSSPP and both device need to use different public IP (ie. different ISP), but not guaranteed to work.

However, if the game didn't use any ports under 1024, it should works, just need to set Port Offset to 0 so PPSSPP will use the same ports used on real PSP.
Not sure which ports used by Peace Walker tho, you can test it yourself.
Usually on PPSSPP you will see red message on screen saying "Failed to bind port xxx" when PPSSPP can't use that xxx port number, if you didn't see these messages it might works (unless there are external factor such as firewall or port-forwarding issue)