I tried a RoadLink on my Motobecane Grand Record with a 9-speed Campagnolo Racing T rear derailleur. I had problems with the derailleur wanting to jump out of gear. I eventually convinced myself that this was because the chain wasn't wrapping enough teeth on the cassette. Campagnolo derailleurs are more sensitive to this than Shimano (for which the RoadLink is designed) because they pivot differently. I was able to make it better by shortening the chain, by I eventually gave up on it for that setup.
So for use with a vintage derailleur, in addition to the total capacity issue already raised, I would say it may work but it also may have trouble.