Donnie
My guess is nothing would happen, if you depart by air, or from another border crossing to the one you went in at, as the Viets should be like the Thais & the computers are not that well linked.
But I'm not a Vietnam expert - Jimoi is the best one there.
Is there anything in your passport to indicate that you took the bike in?
Another complication is that when you took the bike out of Thailand you would have filled in a temporary export permit, that specified you were bringing the bike back into Thailand. On this form there will be a penalty specified for not complying with that condition. Again this should not be a problem, but one day it could come back and bite you if you want to export / import another bike and it is in the computer.
Davidfl
Keep The Power On