mattwnz:
My concern is that someone else may do this and then the vendor accepts it, and I thennever had the chance to put in the offer, because the agent said that they aren't accepting offers before the deadline date. ....
Don’t worry about what anyone else might or might not do, you can only control what you can control.
If it’s a deadline sale, you can put in an offer at any time and the Agent MUST present it. You can make it a condition that the offer is confidential and you want a response in say 24 hours, this would ensure they present it lickety-split.
it’s completely irrelevant what the agent tells you; if it’s a deadline sale then they must present your offer.