Do most of you (and mixers in the industry, for those that may know) record and mix vocals in the same project that you use to create the instrumental? Or do you create the instrumental, import the raw instrumental into a new project, then record/mix your vocals in there?

Until now, I've always believed that it is more "correct" to do everything in one single project, since this way you can mix individual instruments with the vocals as you do your mixing. But I'm starting to wonder if it's more common to do the alternative method, for various reasons: 
 
1) Organization. Instrumentals themselves can require many tracks, many busses, a lot of automation and lots of effects. If you record vocals in the same project, you are adding several more tracks (if you do vocals with harmonization, background, real doubling or chorus as opposed to plugins), more busses, and several more effects. And you are bound to get confused with the massive amounts of effect plugins and tracks and busses and prochannel modules and whatnot. 

2) CPU. Instrumentals already take up a lot of CPU, especially for me since I almost use softsynths exclusively instead of recording instruments. And adding vocals+effects to that can take a pretty big toll on the CPU.

I have a feeling the answer will be the first method... mixing everything in one project, but I just figured I'd ask. Thanks for your input!

Everything in one project if your CPU can handle it. Use track folders to help organize.

Your system should be able to easily cope by having it all in the same project.

Adding audio tracks to a MIDI project that uses soft synths wont burden the CPU. If CPU does become an issue turn the MIDI into audio.  Really what the real CPU users are are plugins not MIDI or audio. Soft synths are plugins too. 

Always in the same project. Folders organize everything. If your CPU is on its limit then just freeze your synths.

However, when I need to do vocals in a different studio, I just record them over a single stereo mix that I bounce out of Sonar. When the recording is done I bring the recordings into the Sonar project.

NEVER, unless you have to for some reason, mix the vocals in over a "finished" mix of the instruments. You can't fit vocals into a mix after it's done, they have to be a part of the mix so make sure you always mix everything together.

I agree with everything said up there ^.  :-)

Thanks everyone for your response! Just to clarify, I do understand that audio files and midi files themselves are not what drains CPU, but rather the plugins that they use. Even for vocal audio, I've got tons of compression, reverb, delay, saturation plugins just for vocals, even after consolidating them via busses. Not to mention v-vocal, melodyne, etc. This is also in addition to all my soft synth plugins and effects plugins used for these soft synths.

Another reason I asked this question is because I know with mainstream commercial music, often times the "producer" is a different person from the "recording engineer" who is different person from the "mix engineer" (who is different from the mastering engineer, but that's beyond the scope of this topic). So I just wondered if maybe the second approach is what that take. But I do agree with everyone here that its important to mix in the vocals with the instruments. Just a thought though!

Just one example of why you should do it in the one project - but there are hundreds more.

You might find that the guitars in your beautifully crafted instrumental simply drown the vocals and you need to drop them by at least 3dB

What do you do if you've only got the stereo mix to work with? Everything you try will be a compromise at best.

All in the same project. I'm running what is now considered to be an older computer on XP32 and I have never had a problem running as many tracks and VST's and FX as I needed. 

I try to emulate a band in the genre which means a 5 or 6 piece band. Most of the projects I do might have a dozen tracks in them since acoustic guitar is usually doubled and I might have 2 lead guitar tracks for convenience.  There is a good likelihood that there will be several vocal tracks as well. 

12 tracks is about average for me. I do occasionally get up to the 20 track count and above.... but not too often. 

Always in the same project. The vocals are usually the most important part of a mix. It doesn't make practical sense to work on them separately from the song. After all, you have to dial in the vocal sound with the rest of the song elements.