I'm pretty sure that you're talking about a fork with a 1" steerer, and how it fits into a 1" threadless headset. If you're talking about fitting a modern stem on to the vintage fork, then I can help you.

You're correct; 1 1/8" is the more standard size currently (before you get to tapered steerers).

The adapter you need to use is a "steerer tube shim" to fit a 1 1/8" stem on to the 1" steerer tube. They're $5 at any bike shop. They come in two sizes, meant to fit inside either a 4 cm or 5 cm tall stem.

Steerer tube shims are safe to use; just make sure all the stem bolts and headset bolts are tight.

Of course, if you're talking about putting the fork with the 1" steerer into a frame meant for 1 1/8", then I can't help you. Headset adapters may exist; I have no experience with them.