Chris, the Vantage I've used usually takes anywhere from 14-28 days before results are visible. It seems to depend on the type grass and the height. You mentioned you mowed the plot before spraying, did you allow any time for new growth????? Usually you need to get at least a few inches of new growth before applying the herbicide. That doesn't mean your spraying will be ineffective but it could cause a less than normal kill-off.

I wouldn't worry about the rain, if it had a hour it should have been rainfast by that time.

Have you noticed the grass growing since you sprayed. If not, the plants are dying and it will take a few more days to see the effects.

Let me know what it looks like in a couple weeks.

Good luck