Singer-guitarist Vince Gill earned his stripes with a variety of bluegrass and country-rock outfits, notably Pure Prairie League and Ricky Skaggs' Boone Creek, before going out on his own in 1984. For far too long, though, Gill looked like a poor man's Rodney Crowell, his obvious hero, whom he emulated in almost every way. Then came last year's ''When I Call Your Name,'' a duet with Patty Loveless that combined honky-tonk angst with a killer chorus in the high-lonesome bluegrass style. That single, from an album of the same title, showed Gill finally finding his own way. It also made him a star, earning both a Country Music Association award, and, two weeks ago, a Grammy. On this new album, Pocket Full of Gold, Gill again plies his considerable vocal and instrumental skills, and Loveless, whose rough-edged tones tend to keep Gill's choirboy sweetness in check, returns for harmony vocals. But, alas, Gill's lyrics never go beyond the surface. From the infectious dance rhythm of ''Liza Jane'' to the upbeat ballad ''If I Didn't Have You,'' his songs seem more like fuzzy images than focused pictures, more mood and groove than substance.