The loop goes: vocal's too quiet, push the fader. Now it's loud but somehow still not clear, and the guitars got weaker. Pull it back. Around the third lap someone says the mix sounds crowded, which is true, and nobody can say why, which is the actual problem.
Here's why the fader can't win this. The body of a voice and the body of a strummed guitar both live around 200-400 Hz. When you push the vocal fader you're turning up its share of an already-congested range - the congestion stays. What you need is for the guitar to vacate that range, and volume knobs don't do that. EQ does.
Put the EQ on the guitar, not the vocal
This trips people up. The instinct is to process the thing that sounds wrong (the vocal). But the vocal isn't wrong - it's obstructed. Open the browser panel, Audio Effects tab, and drag EQ Eight onto the guitar track. You get an eight-band curve you can grab with the mouse.
Then, in order of how much they matter:
High-pass at 80-100 Hz. First band, low-cut mode. Guitars put out rumble down there that no listener perceives as guitar - it just burns headroom and muddies the range the bass and kick own. I do this to almost every non-bass track on principle, and it routinely helps more than everything else in this article combined.
Cut 2-3 dB around 250-350 Hz, medium width. This is the pocket the vocal will sit in. Do it with the full mix playing, not the guitar solo'd. The guitar will sound slightly thin by itself afterward. That's not damage - a guitar that sounds a touch thin alone and perfect under a vocal is a correctly EQ'd rhythm guitar.
Optionally, 1-2 dB at 3-5 kHz on the vocal. Now put an EQ Eight on the vocal and nudge the presence range where consonants live. Keep it small. Needing more than 3 dB up here means the real problem is still down in the instruments - go back and cut more instead.
Judging it
Turn solo off and leave it off. Every one of these moves is a mix decision and only makes sense with everything playing. "Sounds great solo'd" is how you end up with twelve individually beautiful tracks that fight.
And because the session is shared: the guitarist will hear that 300 Hz cut next time they open the project, on their track, that they recorded. Tell them what it's for, or better, do it while they're in the session and play the before and after with the vocal in. I've never had anyone argue after hearing the after. The arguments come from finding the cut unexplained.