OK, the vent tube does not return fuel to anywhere. It is simply a way for air to exit the carb bowl. Just forget about it as part of the problem for now.
The picture below shows my carb. The main fuel line comes straight up from below and flows into the carb inlet. You can see on the photo that there is a "T" coming off of the fuel line just below the rubber portion. THAT "T" is the beginning of the return fuel line and takes excess fuel back to the tank.
If the carb bowl is full, then the float needle will close the fuel inlet. The fuel has no choice except to take the path out through the "T" and back to the carb.
One thing you can do is disconnect the fuel line and make sure the return line is not plugged up. It's simple to do. Just disconnect the rubber portion on the carb side. Place your thumb over the rubber portion to block it and turn the fuel pump on. The fuel will have to flow back to the tank. If it builds up high pressure, then you have a blockage.