If waste is actually spewing out the vent now, it's not likely that the vent is blocked...but if it's only a trickle, it likely is and the tank is seriously pressurized, forcing waste to escape wherever it can....enough that continuing to flush the toilet can burst the tank or create an eruption in the toilet. Don't use the toilet again--use a bucket if you have to!-- till you've cleared the vent. You won't be able to pump out against a blocked tank vent either. When you get to the pumpout, open the deck fitting cap VERY carefully, hanging tightly onto it...'cuz the pressure can easily spin it out of your hand. There's also likely to be a geyser...so be sure that side of the boat is to leeward--and you should be too...and have a hose at the ready.
And btw, if you let your macerator run for more than a minute or two without pulling anything out, yes...you've burnt out the motor.