when the coolant gets hot enough it opens the valve to vent the charcoal canister. It's actually plumbed before the throttle blade so it won't be under enough vacuum to cause a problem at idle. When it's functioning properly, it's open any time the coolant is over roughly 100f so if it's stuck open, it's not going to cause much change in how the engine runs. Yes the system is very approximate, that's why there's an O2 sensor to correct things (albeit after the fact) and why newer cars use more control.
If there is liquid fuel from the BVSV your gas tank has been over-filled to the point of overflowing into the charcoal canister. When the BVSV opened it started sucking in liquid fuel instead of air with a few fuel vapors. I'd guess your lumpy-idle is from running rich.
Coolant is pretty distinguishable from fuel by smell or touch. the likelyhood of coolant leaking out through it is also very low because the threads and bulb that extend into the coolant are all one piece of metal.
"You don't get to judge me for how I fix what you break"
'94 Pontiac Firebird LT1, T56
'90 Cressida, 7M-GE, M5
'91 Toyota Pickup, 22R-E, M5, 2wd