In his last post Jim Cate is correct as to the effect of the Vang except, I believe he ment to refer to the "clew" rather than the tack. Tightening the vang forces the boom down, regardless of the set of the mainsheet, tightens the leach and takes some of the twist out of the sail. If the the sail is continuously attached to the boom, with a bolt-rope in a slot or with slides and shackles, and the sail, designed for that type arrangement, has not been stretched out of shape by, for example, someone taking a reef fore'n aft (slab reefing) without properly fixing the bunt-lines, tightening the vang also flattens the belly of the sail and will do so quite markedly on some boats.
As to whether sail slides load a boom or not, that depends upon the design of a sail and its condition. Some argue that a loose footed main is preferable as it permits the sail to be shaped more readily, similar to the Genoa
. Others argue that since the Main does not have the benefit of the "end-plate" effect of a deck
as does a genoa
, attaching the foot of the sail to the boom reduces reverse-flow under the foot of the sail from the windward to leeward side which reduces the efficiency of the sail, more or less so depending upon the aspect ratio of the sail. In my day, most mainsails were continuously fixed to the boom and most racing sails
were designed with shelves and "flattening reefs", which definately required that intermediate bunt-lines (or reefing lines) be taken up to avoid damage to the sail.
As to whether one can readily relocate a mainsheet, it would be wise to have a qualified engineer
run numbers on a boom as, particularly if a sail is loose footed, moving from end boom to mid-boom sheeting substantially increases sheet loads and can dramatically increase boom loading with bending moments that would not otherwise exist.