I have used the vaquero for a year now and it is a great machine. I would have a hard time buying a machine without manual/automatic ground balance ever again, even though the ground where I live isn't that mineralized. High levels of sensitivity are difficult to obtain without the manual ground balance. If the ground balance is out, high sensitivity will cause a lot of chatter and falsing - especially where the ground is trashy. In trashy ground the vaq is a dream to swing - given the ground balance is spot on. It won't make a peep unless it's over a target within the accepted range of discrimination.
I also read a lot of the recent hype about the vaquero. Much of the hype is true and not exaggerated. BUT, I feel it has been blown out of proportion. The vaquero is good but it is not the 'is all, end all'.
I have experienced depth loss in dry conditions (as little as 5" on a Canadian clad quarter) and have found extreme depth in wet conditions (as much as 16" on a Canadian clad quarter). One thing I have had a hard time with on this machine is the differentiation in target response from bottle caps to pennies/silver. I have as of lately began to hear the difference, but it has taken a lot of time spent with the machine to hear it. With the silver Umax, after a month of using it I knew 75% of the time exactly what I had under the coil, it was an easy one to learn (also my first machine). What turned me off of the silver was it's inability to find small gold - I did find 2 fat gold rings with it but never anything small, which is partly why I moved up to the vaquero. With the vaquero I have found a thin ladies ring and a few days ago an earring. the key to finding gold as we all know is to go low disc and dig the trash. With the vaquero I can set the disc on foil and it will hit good on most all gold rings. To find any small gold I have to put the disc way down around iron and dig lots of junk, if the area isn't trashy I don't mind it. Gold chains are some of the hardest targets to hit on, and anything short of all-metal with the threshold at a mosquito buzz won't find them with this machine, unless there is a pendant attached to it.
The all-metal mode is priceless to me. Sometimes if hunting a park I might get an iffy signal, a quick push of the button and I'm in AM mode, often I can then immediately tell by comparing the signal in disc and AM I will know what I have. A poor signal in disc and a loud/wide blast in AM would signify trash, poor signal in disc and quiet/smooth/medium width signal in AM would signify deep and good chance of it being a good target, poor signal in disc and strong/short signal in AM would signify something small or trash.
You said you had hot rocks? I dug 1 hot rock with this machine and then learned the unmistakable signal a hot rock makes, never dug another since.
Once this machine has been learned, it will be hard to upgrade. There are better machines out there than the vaq, but for the price, lifetime warranty and it's utilization in many fields to out perform many other higher priced detectors - it's worth every penny paid.
I really like this machine and highly doubt it will ever be fully replaced by any other machine out there.
Hope this helps, Good luck finding the right machine for your needs.