Re: Filling Vias?
There are a couple of ways to fill vias and make them airtight. You should discuss it with your board fab.
One way is to fill the via with silver epoxy. This is one way vias are filled under at BGA with via-in-pad, for example. But it also works for vacuum filling. It can be expensive depending on your fab. For fills to which you will solder, the fab will actually plate over the epoxy fill so that the surface of the board is smooth copper and shows no sign of the hole in the filled via or pad.
Another way to fill the via is cheaper and it works - I've used it many times on high vacuum testing equipment. You can tent the via using solder mask. Some fabs use a technique where they tent over the invividual vias you identify with LPI, then they apply your regular LPI solder mask tent over the first set of fills. This provides a very satisfactory vacuum seal as long as you don't have large via holes (<25mils). This method is very dependent on the experience of the fab. Gorilla Circuits in San Francisco, California, USA, does this sort of fill very well ( **broken link removed** ).
If your vias are small, some fabs can plate the vias closed with copper. Copper fill is the cheapest of the methods, but only works for very small vias. It also depends on the equipment and experience of you fab.