Well, it may not be vintage, but I started with a not-too-old 3 valve Couesnon flugel, which I loved, but all I wanted to do was make the 3rd valve slide kick out like any decent trumpet.  By the time I was done, I had moved the bell to the "proper" side of the valves, added a third valve saddle and then changed it to into a thumb rest on the valve block, replaced the standard water keys with Amado water keys, added a fourth valve that I adapted from some french horn or other with the right bore size, (thus a "dual bore", like any decent tuba), changed the beginning of the bell tubing / flare to accomodate the new bore size, plus a bunch more stuff I've probably forgotten.... and.... zzzzzz... it's alive!!!

It now plays pretty freely (much more so than before), better in tune, the 4th valve falls right under one's little finger, and one can use a ton of alternate fingerings, as well as an expand range. Oh yeah, and if you don't like the fourth valve interval, you have a variety of other slides to choose from, another trick I picked up from tubas!