It is probably not that simple Sweendog. To many scenarios when it comes to cleaning. Machines, tools such as rotaries, chemicals, time, heat, agitation, cfm, lift, etc. Maybe there is no sand in the carpet, and if there is much sand, it will not all come out with vacuuming anyway, but it may have become part of the backing.
How many carpets have you cleaned that you turned upside down and beat it? Probably none wall to wall. So even if you vacuumed it first instead of after the cleaning, you will likely get to see that even if vacuumed at any time, you will also see what comes out of the carpet. It is simply not practical to take a wall to wall carpet up, turn it over, and beat it from behind.
However, if it is that simple, then just vacuum the dry soil out after cleaning instead of before.
After all, you have already "emulsified, saponified and separated the oily soils from the carpet fibers that may be holding fibers together or collecting dry soil."
I am not against prevacuuming when necessary, any more than I am against any other procedure, tool, or chemical that will improve the cleaning results. It doesn't matter how you get those results. There is more than one way to skin a cat. No matter how it is done, the cat gets skinned.
I use a fly swatter to kill a fly, but an elephant gun to kill an elephant. Make sense?