Spilled Yeast

Everything was going well, until it came time to pitch the yeast and I spilled some on the counter. The counter was pretty clean and I scraped off the yeast and picked at anything that did not belong and added it to the yeast. In all, I lost about 1/4 to 1/3 of the yeast. Whats the chance that the wort will be infected? It fermented vigorously for 2 days and has now stopped.

Hmmm.... Chances of infection depend on the counter top. The fact that you picked out what didn't belong suggests that it wasn't super clean.
Nothing to do now but wait and see.

For future reference, I keep extra yeast in the fridge. Also, you would have been better off pitching less yeast than throwing unknowns into the beer.

Keep in mind for the future if something like this happens and you don't have extra yeast, you can put it in the bucket with an airlock and get more yeast the next day.
You might check the gravity of the beer. If you think it has lots of sugars left to ferment you could re pitch also.

what kind of beer was it? were you only pitching one vial/smack pack of yeast? i agree it depends on the counter but manually picking out stuff from the slurry siggests that there were probably some funkies that you threw in.

If it's a lower gravity beer then I would suggest bottling/kegging and chilling and drinking asap before something really presents itself...

However, could work out totally fine any you might not get an infection at all...

its a Brewer's Best Belgian Trippel with an OG: 1.081 and FG: 1.017-1.020. I took a reading today (Day 3) and its at 1.030. I bought more yeast to pitch, and if thats the case, should I pitch the entire package, or only half?

You are close enough to the final gravity that I would say the yeast did it's thing fine.
Maybe rack it to a secondary and give it a little time before you bottle.

I would let it keep fermenting for at least 2 weeks.