We pull a vacum on all the valves we machine once the machining is finished and cleaned up we drop in the valves and tap them on the seats several times ( lightly) check for seat contact ring pull a vacum with no springs installed and usually get 15- 20 lbs of vacum if you are using 3 angle cuters and having problems make sure your pilots are compatable with the cutting tool holder we use a Goodson 375 tool holder with the 375 pilots,,Regis also sells the same tooling  works really good but there is always room for improvement

The issue is concentricity of both the seat and valve face. I dial all my valves to ensure they have less then .001 run out while in the chuck and also check the seats for the same. Pilot shape and condition can influence runout when using a tapered pilot. I have many custom pilots with very little taper ( less then .001) to locate the pilot well in the guide. This is for engines I work on all the time. while the manufacture use an interference type valve seat configuration that allows the valve and seat to mash together to seal over time of the break in to seal right.

I find that unless you are keeping the run out down lapping the valves won't fix the problem. Funny thing is valves are machined not ground making the stems flex some during the machining process causing built in run out and depending on what valve grinding machine you have that can also cause issues as well.

It takes a bit of extra time but after you prep a valve to grind put it in the chuck and a dial indicator on the stem about 1" from the chuck and check the run out. If more then .001 the machine's chuck needs some attention.

Personally I have 4 machines all for different engine work. I prefer the Sioux machines with their indexable chuck that allows you to dial out any run out. The kwik Way machines use a 6 ball chuck and the requirement of dressing the tip before chucking the valve is a must to reduce runout at the face.
There is some very simple fix's for both machines that don't cost an arm and leg.

And also pilot runout can be an issue.
Are you using HSS or Carbide pilots.While carbide is prefered for ridgidity on anything 7mm and smaller HSS will do the job for the larger sizes very well as long as they are straight and run true

Personally I dial indicate every valve stem while in the refacer and check every seat just to be dead on every time.
I use a Sioux refacer and it has the ability to adjust the chuck to various stem sizes. The 680- 2001 and 2075 have 4 position chucks that allows the user to correct run out so the valve face is within .0002 .0005 if directions are followed on proper use of the mqachine.
I got the procedure from the sales rep here back in the 80's and never changed it's use. 
Things guys forget is valves are not centerless ground they are machined and there fore have some built in runout and 95% of shops do not concern themselfs with this issue saying it takes too much time but in reality it takes an extra 1/2 hr over the course of the complete machining operation.

The bouns is better vaccum signal for computer controled engines thus tighter guide clearances can be used for better oil control. Gone are the days of OEM interfearance fit valve jobs requiring 10,000 miles to fully seat. Customers want instant sealing so they have no running issues with vaccum signal sensors.
They are not adjustable and as such I find Fords and almost any Chrysler product as well as imports like Toyota and Honda as well as Mitsubishi.
If I remember rightly there is some TB;s regarding valve sealing on some engines that does not show up on a typical compression test but I do see a few more shops now with dual gauge leak down testers to help figure out cylinder sealing issues.
Valve seat and face concentricity is the most important thing in a good V/job and with the right tools you can go from average shop to Rock Star in no time.

I dont think a hand vacuum would be very easy to use. Plus none of them will hold a vacuum (it gets past the guides) We have an air powered rig from goodson that we integrated into our seat and guide machine.(I think Newen also sells or sold one for their machine). For single or dual valves we expect to pull 24 inches of vacuum. For big diesels with cast in intakes where you are checking all the valves at once we expect to see 15- 20 inches of vacuum depending on the number of valves and overall size of the head. We check ours on the machine with the valves and seats wiped clean and the valve sitting on its seat of its own weight.