Not sure how good a joint you need. 

A lot of copper gets 'sweat' (soldered) for plumbing and gets brazed for air conditioning and high pressure.   I have sweat a lot of copper pipe using steel wool to clean it.   Some like a green pad.  Clean to bright copper.   Make sure it is grease free.   I like a caustic.  Many like a solvent. 

I have never vacuum brazed copper.    I have never seen the need to do so.   We think of vacuum brazing as expensive.  We use it when nothing else will work.   We start with a torch, induction or a belt furnace with flux depending on part size and batch size.  

Were I you I would get a propane torch and some copper fittings with solder and flux and experiment a little.  Home Depot and similar have them.    Maybe $30 would get this done.  If you are at a university ask maintenance to show you how to do it.   

We have little induction set up here where we show people how brazing works.   Nothing is as instructional as actually doing it yourself. 

Theoretically, you shouldn't have oxides in a vacuum.   Practically that is what you will decide. 

BCuP alloys are Phos -Copper or Silver, Phosphorous and Copper.  Try Prince & Izant, Handy and Harman or Lucas Milhaupt.   We don't braze copper.