Does a request for passengers to open vents indicate pressurization issues or equipment failure?
I have occasionally asked passengers to do the same thing.
I have also asked for window shades to be lowered.

The reason is usually because the APU was not working and the ground air conditioning was not doing a good job or was unavailable. 

Without the APU the aircraft's airconditioning can not work and engine starting relies on a ground air source to provide air pressure to spin the engines for start up.  

When using ground air for starting, the ground air conditioning  is disconnected first and it takes very little time for the cabin to become uncomfortably warm.