You should seriously consider staying a little bit longer. Correct me if I'm wrong, but by leaving a few weeks early, you're passing up $300 a month when you hit 65. Think of it like this. If you live to be 85, that's $72k you're passing up on by leaving a few weeks early.
Yes, you are vested in your pension after 5 years of service. However, you have to be there a helluva lot longer than 5 years to get the $300 per month.
I've been with the company ~8 years now. And, IIRC, my most recent pension statement said I would earn maybe $200 a month if I work to retirement.
This, of course, assumes that there will be a pension fund left to draw from ~20 years from now.
Back to the OP - stick it out a little longer to make sure you get vested in the pension then put in your notice.
I've read here that once you're vested, you may be able to cash it out and take it with you when you leave the company.