Heh, you know I’m not sure about the “X” part, I see it recommended like so to the point where I’m just going with the crowd now 
Best I can tell you really should be fine with express even up to release, but hate to really state such a thing definitively. Even with some of the people I’ve worked with we’ve had differences where they’d find some feature indispensable and I’m sorta “meh.”
Never did try Unity with VS, just mono. But seeing as content I am with C# otherwise in VS I can only imagine that it would be real nice. Plus if you are doing any sort of ASP you really can’t beat Pro. (Stick with php tho if you can). So maybe it’s being able to justify the cost for numerous systems that makes the decision. For only UE4…eh, really tough choice there, like maybe if one could write it off as a business expense or something.
Edit: oh the VAX really is just a convenience thing. It doesn’t make your programs run any Bette (best I know) but will be very nice during coding. Otherwise what you get is confusion as the system keeps highlighting bad code, except it’s not bad, or maybe it is, you just need to wait a minute until VS figures it out…except when it doesn’t figure it out until you add new functions and it rereads headers…