Need to know the location of the vacuum leak in order to answer this.
Check for vacuum leak
You have a small "EVAP" leak. Start with replacing the fuel cap.
Check the 96 Blazer for any vacuum leaks in the system. A vacuum leak can cause the air flow to only come out of the heat vent.
You failed to mention the model year but a large vacuum leak could do that.
What does it cost to repair a vaccum leak on a 2001 deville
A vacuum leak could do that, also low fuel pressure or a dirty fuel filter.
Start with a visual, inspect all vacuum lines for problems.
Depending on where the leak is you just replace the vacuum hose or the bad part the hose is attached to.
First you have to locate the source of the vacuum leak, then determine the next step how to repair it.
you may have a vacuum leak somewhere.
when i was tring to find a vacuum leak on my s10 i just used carbarater cleaner. and sprayed it around all the hoses and if it reves up that's where it is leaking.