I found a couple of great deals on Vayama, however, the deals are with the Star Alliance and I won't find out what airline I'm flying with until I book my ticket. Is this advisable? I'm going to London in late May. Star
Vayama is a well known booking engine.
You do not mention what airport you are departing from? Also, does Vayama show you the departure and arrival times?
With a little bit of investigation you may be able to figure out which of the Star carriers it is. If flying out of IAD, it is most likely to be United.
Other options could be US Air or BMI or Lufthansa. Again, difficult to say without you providing more information.