My translation from Russian:
Fill the vial with nitrogen, then immediately close it with a stopper and crimp an aluminium cap on.
The Russian original uses the word "завальцевать", which is basically "to crimp" here, and I want to retain it in the sentence and not just write "cap with an aluminium cap" or something like that.
Maybe it's not very natural-sounding and one should write something like
... and attach an aluminium cap using a crimping instrument.
A well-crimped vial looks like this: