What did you think we were talking about?
Of course Darth Vader, being that seven-foot-tall bionic killing machine, has a definition of "fire" that is more in line with the common definition of "strangle to death". If Grand Moff Tarkin hadn't told Vader to stop, he probably would've thrown Motti out the goddamn window like Patrick Swayze in Road House. As it stands, we're pretty sure the Admiral had been promoted to cleaning toilets on the detention level by the time Luke blows the place up at the end.
The point of the scene seems to be that somehow Motti didn't know Vader had magic telekinetic strangling abilities. But even so, and even if an Imperial officer like Motti somehow also missed the memo on what not to say to a Dark Lord of the Sith, that still means that he thought Vader only had about 37 different ways to instantly kill him instead of 38 (most involving a lightsaber neatly carving through his groin).
What we're saying is that Motti was hired not for his tactical acumen but purely for his ability to sneer.