Price was all wrong
I agree that Vincent Price was thoroughly miscast (as a babe magnet gigolo). Not conventionally attractive, effete and annoying in his determination to push that half-assed southern accent. Fox was constantly pairing him with Tierney. He hit a similarly bum note as her fiance in "Leave Her to Heaven". Only their final teaming (in "Dragonwyck) worked. There he was supposed to be domineering and unbalanced and - as such - was quite effective. I think Price's finest performance was in "The Song of Bernadette" in '43. He was brilliant in it and definitely deserved a supporting actor nomination that year.
As for the role of Shelby in "Laura", here's a list of ten actors of that era who - to my mind - would have been much better fits: Louis Hayward. John Sutton, William Lundigan, Franchot Tone, Richard Denning, Hurd Hatfield, Craig Stevens, Don Castle, George Reeves and Farley Granger.