Let's not be silly about exploding diesel
Assuming the fumes are not hot, you will find it very difficult to ignite them with a spark, or flame. Diesel
is not volatile like Gas is, that's why we prefer diesel on boats.
Secondly, the sparks you see from the motor are exposed to atmospheric pressure, they are not exposed to the vacuum side of things and will not be exposed to fumes that are concentrated enough to ignite. Assuming you are in the room, and breathing the same air to which the sparks are exposed, you won't cause an explosion. There will be fumes coming out of the exhaust
side of your vacuum, but they are extremely unlikely to ignite without being heated.
Diesel will only explode under very special circumstances.
Now, since you may be an American. (No insult intended) I am obliged to say that I take no responsibility for your actions, and do not recommend any action on your part. The information given here is for educational purposes only.
I'd be interested to know if a vacuum would stop a leak as you describe, and will try it myself as an experiment
. I'd be careful of not collapsing the tank, even at low vacuum.