We froze ham slices in vacuum packs in January and they really turned red in color. Are they still good?

Ham is a cured meat. The curing process can be carried out in a variety of ways - smoked, salted or chemical, or a combination.

I don't know why it turned red but I speculate something, perhaps a chemical used in the curing process has been drawn from the meat to the surface by the vacuum.

Cured meat in the freezer is very unlikely to go bad so it will certainly be safe to eat, though you could find the taste is altered.

Properly cured meat shouldn't need freezing unless you want to store it for a long time and then wrapping it should be sufficient.

A widely travelled friend told me in rural France ham is dry-cured and in many French kitchens a visitor will find a large ham hanging from the ceiling, sometimes wrapped in mutton cloth or cheesecloth. Sometimes a slice or two is taken from the ham where it hangs and sometimes it is taken down.

That ham hangs around for a relatively long time before it is exhausted and no one becomes ill.