Interupt vector is adress where your program will jump after interupt has been trigered.
For 5'th interupt it is 5Bh. So you should write something like that:

ORG 5Bh
' do what you want in interupt
RETI

If your interrupt routine is long, insert only jump to the routine, otherwise it can overlap other interupt vectors space!

About flag. Usaly flag is bit to indicate saomething has hapened. In your case, flag will be SET after INT5 has been trigered and program will jump to interrupt vector. If you will not clear interupt flag, after RETI program will jump again to INT5 interupt vector, thinking, that it was trigered again.

Yep... begining sometimes is hard... ;)