Is it safe to remove /vm/swapfile0? I found a file of size 1 GB in /vm/swapfile0, that has not been accessed in a long time, according to last access date value in ls -l. 

My assumption is, that this file is left over from an older macOS version. The last access was probably before I upgraded to 10.12. Can I safely remove it?

I also found some files of size 1 GB in /private/var/vm. These have recent access times; both sleepimage and swapfile1 today and swapfile0 two weeks in the past. I think these are in active use.

One side question though: sleepimage has a size of 1 GB, although I have 4 GB RAM installed. How is that possible? Other threads on the topic of sleepimage suggested that the size would match that of the RAM installed in my Mac.
Boot up in Recovery mode and use the Terminal to remove the swapfile0 file. That is fully safe to do.