More Is Better
Remember Folks, as you shop for a vacuum pump, CFM flow is more important for turners than inches of mercury.
When you vacuum chuck a piece, you're using atmospheric pressure to hold your piece, but that piece is wood, and will, to greater or lesser degrees, be porous and will leak air into the system. You need a pump with enough flow to both maintain the partial vacuum and make up for the leaks. Therefore, a vacuum pump that may be great for a veneering system at 2 cfm pulling 28" Hg. will be quite unsuitable for turning wood, but a pump moving 6 CFM while only reaching 15" Hg. may be just fine.
To further illustrate, remember that the larger the diameter of the chuck, the less "vacuum" you need to hold the piece. An 8" chuck has the ability to develop more than 3,000 lbs. of pressure on your workpiece at full (30") vacuum at sea level. Thus you won't need more than 10" Hg. from your system to hold that piece in the chuck with 1,000 lbs. of pressure. However, since that 8" chuck depends upon a good seal around the 25" circumference in contact with the wood that can slightly bend and wiggle during cuts, the more flow your pump will tolerate, the stronger and safer a fixing you will have. That calls for more CFM. Indeed, the original "vacuum chuck" lathe rigs used shop vacs pulling 30-40 CFM but only developing 5-6" hg.
As mentioned, there are 4 CFM vacuum pumps available at reasonable prices. I would submit that you go no lower than 4 CFM, with 5, 6 and above being preferable for a turner's application. I use a pair of 2.5 cfm AC compressors in my rig. They are reasonably tolerant of leaks, but I watch my vacuum gauge for fluctuations.
Some day I'll get a "real" vacuum pump in the 6 cfm./ 25" Hg. class.
