In a case like that, you should probably consider using an air
You see, the whole idea behind vent piping is to prevent the partial
vaccuum that can be created behind draining water from sucking the water
out of the drain's P-trap. By connecting an empty pipe (called a
"vent") to the drain pipe a short distance downstream of the trap, air
will be sucked into the drain pipe behind the draining water instead of
the water in the p-trap being sucked out.
An air admittance valve (also called a Studer valve after the company
that first marketed them) is really nothing more than a spring actuated
check valve. You mount it on the drain pipe a short distance downstream
of the P trap. If a partial vaccuum ever develops in the drain pipe
behind the draining water, the air admittance valve opens allowing air
to rush into the drain pipe behind the draining water, thereby ensuring
that there's no risk of the water in the p-trap getting sucked out.
You see, the worst that can happen if the water in your p-trap gets
sucked out is that you have the smelly air in the sewer coming up
through that empty trap to stink up your bathroom. That sewer gas isn't
pleasant, but it's not poisonous either. So, it's best to have your
drains vented, but where venting isn't practical, most jurisdictions
will allow you to use an air admittance valve instead.
Google "air admittance valve" or "Studer valve" and read all about them.