kito:
Since no one has replied to you boom vang attachment method question, you might do your own internet search. Lot's has already been covered many times. Below is one link.
I myself had already thought about suggesting the method that is described towards the end of the url.
To mount a moderately robust pad-eye, drill/tap at the forward side base of your mast for couple of say 1/4" bolts. Then run a loop of (say dyneema/amsteel) through it, around each side of the mast and connect to your lower boom vang block. The pull forces on the vang are at say a 35-40 degree tangent, so really the pad-eye mainly serves to prevent the loop line from riding up the mast. Of course, you would have to determine if anything else you have along the lower foot or so of your mast might interfere.
Attached is a picture of my boom vang set-up. (Admittedly it is a bit of over-kill, but I had the extra block and also the boom already had two bails I could use.) As you have observed, unused ex-mainsheet blocks can serve well for boom vang use, which mine are. I added my vang in 2007 when I bought my 1980 Cherubini Hunter 36 because for the previous 27 years, the boat had been sailed without one! I did add a bail at the bottom of my mast. But also because I wanted attach points for several other rigging modifications. (Cunningham, mainsheet and vang blocks for re-routing back to cockpit, and better leads for the reefing lines.) If it was only a boom vang that I had been considering, I probably would have opted for the simpler solution of a pad-eye.
Anyway, just a suggestion. Should work. Inexpensive. And other than two new rather small holes added to the mast base, no real serious modifications.
One downside is that the loop line will rub on the side of the mast eventually wearing through the paint.
http://www.cruisersforum.com/forums/f116/boom-vang-attachment-advice-38173.html