If you have the VPOP3 Mail Collector setting Leave Mail On Server enabled, then VPOP3 will leave your messages on the ISP POP3 server for the specified number of days after downloading it.
If, during this time, you want to make VPOP3 download those messages again, then you can do so.
In VPOP3 v5 or later, go to the Settings → Mail Collector → (collector) → Messages tab. You can select the message(s) you want to re-download and check the 'Get' box for them. (Or check the 'Get' box in the table header to get all the messages).
If you are using VPOP3 v4 or older, see our Wiki article