[RogueRose]

I was considering using vacuum to help speed evaporation of some sulfate and or nitrate solutions as it will reduce temp needed as well as give the type of crystals I'm looking for.  The thing is that when doing this a lot of vapor will have to pass through my vacuum (compressor) and I'd like to avoid this.

Is it possible to use another vessel in line with the process between the heated evaporation vessel and the vacuum pump.  This second vessel would be filled with some water with the in-flow line running under the water and an exit line, located at the top of the vessel, running to the vacuum pump.  The middle vessel could be temp controlled down to 35F if that would be beneficial at all. 

In doing what I explained I would think that any vapor would condense in the middle vessel with only minimal vapor exiting that vessel.  I could add some kind of desiccant filter tube in-line as it exits the second vessel if no moisture (or absolutely minimal) can be tolerated.  I was thinking of using something like anhydrous CaCl2 for this as it can be easily dried and is inexpensive and available. 

Does this sound like a plausible and functional setup and or is there a better method of doing this?

[Arkcon]

Yes, on some level, your description is common, although in some ways, uncommon.  Lets try to work with it.

I was considering using vacuum to help speed evaporation of some sulfate and or nitrate solutions as it will reduce temp needed as well as give the type of crystals I'm looking for.

Vac drying of crystals is done.  However, you've left off some specifics.  Tell us, what sort of crystals you're getting, and what sort of crystals you want, so we know you're not headed in the wrong direction.

The thing is that when doing this a lot of vapor will have to pass through my vacuum (compressor) and I'd like to avoid this.

That's what's meant to happen.  Consider, if you just turn on the vac pump, for as long as its on, air is going through.  What vapor do you mean?  Solvent, or water vapor?  As oil gets hot, vapor, water and especially solvents go out its vent.  Although the oil eventually gets contaminated.   Note: I'm describing an oil pump, there are other types.

Is it possible to use another vessel in line with the process between the heated evaporation vessel and the vacuum pump. 

We call that a trap.  Its commonly used.

This second vessel would be filled with some water with the in-flow line running under the water and an exit line, located at the top of the vessel, running to the vacuum pump.  The middle vessel could be temp controlled down to 35F if that would be beneficial at all. 

That's called a cold trap.  And it can get colder if we want -- filled with dry ice, or refrigerated, even down to -70 C, or below.

In doing what I explained I would think that any vapor would condense in the middle vessel with only minimal vapor exiting that vessel.  I could add some kind of desiccant filter tube in-line as it exits the second vessel if no moisture (or absolutely minimal) can be tolerated.  I was thinking of using something like anhydrous CaCl2 for this as it can be easily dried and is inexpensive and available. 

Sure.

Does this sound like a plausible and functional setup and or is there a better method of doing this?

You wrote a giant paragraph to reinvent the wheel.  Try a Google for 'vacuum drying' or 'rotovap' and see what exists, and then see how you application fits, and what might need to be changed.