Unions can be disassembled and reassembled. I usually do not waste too much time trying to get a union to seal; I would rather replace it with a new one. I am assuming that you are having problems with a metal union with ground faces.
I put a little pipe dope or other lubricant on the union nut so that it can be tightened smoothly and more easily. I am not shy about tightening the bejesus out of the nut; the worst thing that can happen is that the nut distorts and then you may have to replace the union anyway. I have been known to put pipe dope on the ground faces of the union when I have had problems with them sealing (Shh don’t tell anybody, I do not want the purist-plumbing-police to come after me).
Use teflon tape or pipe dope on the pipe threads like any other pipe fitting. My experience is that cheep unions waste too much time. I hate imported fittings. At 1-1/4″, I might consider companion flanges rather than unions. Vacuum leaks are a PITA. Are you sure you know where it is leaking?