I use FL CH until above 10,000 feet. I have had VNAV work fine from as low as 3,000 feet! VNAV has worked at below 10,000 feet on my last 3 flights, at progressively lower altitudes, with 3,000 feet being the last one.

With that last flight, I'm pretty sure that I had the Altitude Hold indicator set to 10,000 feet, while all the others have been set to FL300 or above.