They're the exact same game, exact same content. The only difference is the VR version needs a VR headset (obviously) and this one doesn't.
Not entirely the same. VR version has you dual-wield the guns, but the character also moves alot slower. So, same levels, but they do play differently.
Honestly, I really enjoyed the flat version, but felt letdown by the VR version (bought it just because I really liked the flat version and the dev's previous game that they didn't release on steam). The movement was so slow in VR (I'm assuming for people that get sick easily), that it made it so much harder to play and IMO less fun.