WLP820 is a crap yeast that should not be sold. There, I said it.
Seriously, WLP820 is known to have a very very very long lag time, and it doesn't produce a better beer than any other of the lager yeasts on the market, so.... yeah.
Pitching just one vial without a starter just made matters much much worse. This beer might take 4-5 days to start fermenting, and by then, it might be wild yeast that does the job, not the WLP820.
My best recommendation is to immediately purchase two packets of the dry yeast called W-34/70 and throw that in. You'll then get good fermentation within about 36 hours, and the final beer will taste great. I guarantee it.