My commander is Strefan, Maurer Progenitor. If I use his ability to bring Crossway Troublemakers onto the battlefield, would any declared attacking vampires I control still get the keywords? Would Crossway Vampires get the keywords, even if I didn't declare them as attackers?
Does Crossway Troublemakers have to be on the battlefield during the beginning of combat to affect other vampires?
All attacking vampires, including Crossway Vampires, will get the keywords
Crossway Troublemakers has a static ability that applies at all times it is in play, but only to creatures that are, at that moment, attacking vampires you control. Crossway Vampires is, from the moment it enters the battlefield in your scenario, "tapped and attacking", and therefore qualifies.
Now, if it was worded similar to Strefan's ability, as in:
Whenever a vampire you control attacks, it gains deathtouch and lifelink until end of turn
then it would not apply to either Strefan or Crossway Vampires this turn, as that is a triggered ability that triggers when attackers are declared, and Crossway Vampires was not on the battlefield during the declaration of attackers, and thus does not "see" the declaration in order to trigger.
No, Crossway Troublemakers don't need to be on the battlefield when attackers are declared for its ability to grant keywords to attacking vampires.
The ability on Crossway Troublemakers is a static effect that will affect all your attacking vampires as long as Crossway Troublemakers ins on the battlefield. If the Troublemakers are removed the attackers will lose deathtouch and lifelink.
If it had been a triggered ability that needed Troublemakers on the battlefield before declaring attackers it would have had to have been written as:
Whenever one or more vampire creatures you control attack, they gain deathtouch and lifelink until end of combat.
Whenever a vampire creature you control attacks, it gains deathtouch and lifelink until end of combat.
The difference between to two doesn't matter most of the time, one is a single trigger and the other is a trigger per creature, mostly this only matters for how easy the trigger is to counter. Worded this way Troublemakers needs to be on the battlefield when attackers are declared, and only vampire creatures declared will gain the keywords (IE not put into the battlefield tapped and attacking by Strefan). The abilities granted this way would also have survived removing the Troublemakers.