I've often found one way to use a vintage transfer is to spray on a couple coats of Clear Coat. Krylon or Rustolem (better) and let it cure. This forms a "shell" of sorts and allows the decals to come off in one piece.
I recently did this with some circa 1940's waterslide transfers of french bands. The first couple I tried to put on just fell to bits. Used clear coat and then went on without a hitch.
One not, don't use spray fix clear or other temporary art clears. Use an actual enamel clear for paint.
You can also use matte or satin if you don't want it to look to "new".
San Francisco, CA