Quick Summary:
Civ 5 + DLC without VP: 6/10
Good solid game, ok AI with only a few flaws.
Civ 5 + VP: 9/10
Though I disagree with some of the changes VP has made. (removal of the second UU is the main one) Overall the game has become worlds better.
Civ 6 + DLC: 4/10
Though they improved in some areas over Civ 5, in many areas they have taken several steps backwards.
Detailed Information:
Civ 5 + VP is probably the closest I have seen to a perfect game of it's type. Things such as Vassals, improved AI and over all more detailed, but cleaner systems make it just amazing. It's not perfect in my eyes. There are certain creative differences I have with the direction. The biggest is the removal of the second UU. I also disagree with Gazebo on things like defensive wars (not) being seen differently and that enjoyment should be more important than having things perfectly balanced (edit: to be clear, I feel enjoyment trumps balance). (though I notice the team has shifted more towards the enjoyment side with some recent changes). I also prefer a more "realistic civ simulator" rather than a "global chess game". Something that VP seems torn with, it's changes seem to flip back and forth between both schools of thought. (neither one is wrong, though I do enjoy the balance between that VP tries to ride)
Overall it's a great mod. It has extended the life of Civ 5. To give Firaxis credit though, they made a flawed, but decent base game.
Civ 6 though has some things going for it, such as civs that act and play fairly different from each other, improved city states, barbarians that can actually challenge you if left alone, and a casus belli system, it just drops the ball so hard in other areas that it probably will never recover. Civ 6 plays and seems to be designed by a corporate committee. This includes what decisions they make to ensure the maximum profit, both now and with future DLC. Making a good game is a distant second. I believe they intentionally make their game lacking in areas, so people playing it are constantly hooked into buying more DLC to make the game closer to perfect, but it won't quite get there because there is just no money in doing that. They also work on things that "market well". Having a really good AI that can match a human player doesn't market well. Having flashy new feature like a brand new civ or.... ugh... vampires.. well that can be shown quick in 30 seconds video ads. For me, the biggest back step in Civ 6 is the world congress. It's just so dumbed down with minimal input from the players. World congress Emergencies are a cool system, but that's it. The concept of a world congress leader and actually putting forth proposals? Nope, gone. I also hate the new cartoon style of the whole game. I could live with the new leaders, but the map looks like a "Clash of Clans" sort of game. Thankfully they made a developer mod that puts Civ 5 map graphics into the game. Though while playing it, I kept thinking, why not just play Civ 5 + VP?
I would be really interested in knowing what the developers and Fraxis as a whole thinks of VP. I hear rumors of a certain contempt for it, but of course these are only rumors and should never be taken seriously unless proven.