First - what do you mean by a vent? Is this a connection open to atmosphere with a block valve? If so, you'll want to check with your process and client people on when that valve is operated. It is highly unlikely you will be dealing with design pressure in that case.

If you have a vent line open to atmosphere that has a rupture disk set to fail at a set pressure - you are going to want to find that set pressure to use as the basis for your pressure thrust.

In any case - the load will be subject to occasional loading requirements. If it is a vent to relieve pressure for vessel entry, the pressure will rapidly drop off as contents are vented. If it's a rupture disk, popping one is going to catch people's attention and things are going to get shut down fairly soon to keep from venting product indefinitely.

Also in either case, "design pressure" is not the number you should be concerned with.