Vocals usually more on the right side than left?

I noticed this too! Not only with headphones, but with speakers as well. I suspected there was something wrong with my hearing at first, but reversing the speakers / headphones resulted in shifting the balance to the left side.

However, this only occurs in certain recordings. Strangely, quite a number of recordings seem to be "affected", and it's not only old ones from the early days of stereo. A lot of recordings from the 60's have a very hard channel separation, and usually the louder instruments (guitars, drums) appear to be on the right side.

This is really weird. Luckily, crossfeed can be used to counteract it a bit - I listen with crossfeed enabled almost all the time.

While it's certainly possible that our ears aren't perfectly balanced, I did read that many sound engineers slightly skew the vocals to one side or the other.  This is to open up the midrange a little so it doesn't sound so crowded.  It's so all the sounds aren't fighting for space.

For the longest time I thought I had worse hearing in my left ear because of this and the left speaker in my old headphones wasn't as loud. Really had me thinking that my hearing was going in one ear.

You are not crazy, I could name a few songs of the top of my head that do this. I usually have 1 IEM in an ear when I work and sometimes I'll get only music on one side and other times there will be vocals, usually my songs only do it in the beging ,but yah I thought I was crazy at first or my IEM's were broken.

If in doubt, I'd just do a mono downmix and see whether the effect is still there. It's not like cans or volume pots never had any imbalance.

BTW, sometimes voices can also wander around somewhat, especially over IEMs. A bit of crossfeed tends to do away with that.