Which container to use?
No, you don't want a vector, for a number of reasons:


There is no "vector" unless you add extra (unsupported) code.
Vectors use dynamic memory. Arduinos don't work well with dynamic memory.
It's pointless anyway.


Just allocate an array big enough to contain enough timestamps. In a 10 second period, if you get more than 100 beats, the person you are measuring the heart rate of is most likely in a noticeably "unfortunate" state. That's 600bpm. That's entering panicked mouse territory.

Even with that extreme number of beats an array of 100 32-bit values is only 400 bytes.