Connect your Vita to your PC via VitaShell. Copy the .vpk file anywhere into your ux0: partition (create a folder called VPKs to keep it organized).
Launch PKGj. It will automatically download the necessary databases for games, DLCs, and updates. ps vita roms vpk upd
Before you can work with VPK files, your PS Vita needs to be "homebrew-enabled." Here's a brief checklist to get started: Connect your Vita to your PC via VitaShell
Navigate to ux0:data/ using VitaShell's file browser. ps vita roms vpk upd