Don't do ANYTHING involving the vent DURING pumpout! Spraying water at it may not completely block it, but it reduces the amount of air that the pumpout pump can pull in enough to prevent it from working efficiently and and the only thing it can do to anything IN the vent line (bits of waste from a tank overflow, insects, etc) is get it wet...a fine mist can't flush it out. And if you still have a "vent" thru-hull it can't even do that much, although it might clean out the thru-hull enough to prevent a total vent blockage.
I doubt if any of you realize that holding tanks require maintenance...they need to be flushed out 2-3 x/season--and especially last thing before winter or other extended layup--to remove any sludge It's easy to do: Put 4-6" of water into it VIA THE DECK PUMPOUT FITTING 'cuz that sends the water into it at the bottom to stir up any sludge and hold it in suspension so it can be pumped out. Pump that out...repeat, repeat, till you're pumping out clean water. If you have an overboard discharge pump, repeat one more time with it running to flush out the macerator pump and its plumbing. It would also be wise to completely rinse out the tank with detergent (Dawn is great for this) and water to clean the walls.
At the risk of appearing self-serving, I also suggest you consider picking up a copy of my book (see link in my signature below,, just click on the title). Its title (my publisher's idea) is a bit misleading, 'cuz although it does deal with every source of odor on a boat, their causes and cures. it's actually a complete "marine sanitation system 101" manual that includes everything a boat owner needs to know about US regs, equipment plumbing, operation and maintence--preferably preventive 'cuz prevention is always easier than cure and you can do it when convenient....the need to cure never happens when it is.
(end of shameless self promotion)
--Peggie