which are really written in a complex way without any obvious reason.
I beg to differ, but YMMV.
1) What means " Place NVA and VC Bases in the highest- and US Bases and Troops in the lowest-numbered empty spaces " 1.4.1 page 3 ?
See the playbook, page #2, for a picture of how a full game is set up; observe how NVA and VC Bases are organized and look at the empty Available Forces boxes on your map; observe how US Bases are organized and look at the empty Available Forces box on your map.
2) For the US the Support+Available indication in the start of a full scenario game is 38, probably am counting wrong but in the set up the total support is 15, the rest 23, the only guide I found was the image in page 2 of the playbook where are 20 troops + 3 irregulars, but why ?
That image shows 21 Available US Troops and 2 Available US Bases, i.e., 23 pieces.
3) How the US Led- Irregulars box is used ?
Keep forces Available for placement in the Faction’s Available Forces box (or the US-led Irregulars box).
4) What means Low#=Available Pieces ? Is written in the US Available Forces box
Rule 1.4.1, again, just after the few words you quoted, says:
to show the number of on-map Bases and Available US Bases and Troops
and thus the lowest visible numbers are the number of still available US Troops and Bases.