I certainly agree that the quality of vac-formed sails varies from kit to kit. In the mid-to-late sixties the people at Revell apparently put some real thought into the subject. At the moment I'm looking at the long-extinct Revell Golden Hind. (I got one courtesy of our good friend Big Jake, shortly before his tragic encounter with Katrina. My intention is in fact to build it - but no promises. I'm notoriously unreliable that way.) The sails that came with it had some cracks in them and got thrown out, but in some ways they really weren't bad. The designer had done a beautiful job of shaping them with subtle, carefully-shaped "billows," as though the ship were on the port tack, and the "seams" in them were barely noticeable.
I've got two fundamental problems with vac-formed sails. In the first place, the vac-forming process, by definition, only works in one direction. A raised feature on one side of the plastic has to have a corresponding depressed feature on the other. In some kits (generally on small scales) the designers insisted on molding things like reef points, buntlines, and leechlines in with the sails. Those "ropes" might be made to look acceptable (barely) on the front of the sail, where they appear as raised lines, but look ridiculous from the back, where they appear as grooves. Many kits also overdo the "seams" between individual cloths, producing rows of oversized raised lines on the front and utterly un-prototypical grooves on the back. The worst offenders, though, surely are the manufacturers who try to make vac-formed "furled" sails. Such "bundles" just might, with some careful painting, be made to look reasonably convincing from the front, but the "bundles" are hollow. The appearance of such a "sail" from the back is like nothing ever seen on a real ship.
My other reservation about the system concerns the fact that real sails, with extremely rare exceptions, are not of uniform thickness. Hems, reef bands, mast cloths, top linings, and various other features make parts of the sail much thicker than others. When light strikes a real sail from behind (i.e., when the sail is between the observer and the sun), those thicker portions of the sail look conspicuously darker. And other features of the ship - the spars, the other sails, and the lines of the rigging - show up in silhouette. (You may not have noticed that, but it's true.) A well-made vac-formed sail does a reasonably effective job of filtering the light (and does, sometimes, produce that silhouette effect), but the molded-in reefbands, hems, etc. look phony when backlit.
Painting the "thicker" parts a darker color won't do the trick; the differentiation between "thick" and "thin" sections should only be obvious when the sail is backlit. I've played around occasionally with the idea of trying to stick extra layers of styrene (or some other material) to the surface of a vac-formed sail, to represent the reef bands, hems, etc. That might be possible; frankly I've never been able to convince myself that it would be worth the trouble.
The most effective "set" sails I've encountered have been on either very large-scale or very small-scale models. The big ones I like have had fabric sails made from individual cloths, and the small ones have had paper sails. People like Donald McNarry and Philip Reed have made some beautiful small-scale waterline models in scenic settings with paper sails, photos of which can just about convince me that I'm looking at pictures of the real things. I've also seen a handful of utterly exquisite models with sails made from wood. To my own, personal eye, though, there's something odd about a full-hull model, mounted on a stand, with its sails bellying out. Where's the wind coming from - and why isn't the ship moving?
I think I'm just as sensitive as the next person to the image of a frigate or clipper ship bowling through the sunlit blue sea under full sail, or charging around Cape Horn under close-reef topsails. But I decided long ago to leave such images to the marine painters and novelists. To my eye a model under bare poles, or (better yet) with furled sails, conveys a sense of pent-up potential energy that satisfies both my old-fashioned, romanticized fascination with ships and a desire for realism in scale modeling. I'd rather close my eyes and imagine what that ship looked like under full sail than spend months on a set of sails that are almost - but not quite - convincing.
That, however, is an entirely personal and individual way of looking at the subject. One of the great things about this hobby, to my notion, is the wide range it offers for personal taste and interpretation. To each his (or her) own. Anybody building a ship model has my most enthusiastic encouragement and blessing to approach the problem of sails in whatever way he or she thinks is most appropriate.