Haldane has been training with the League Two side for the past week and has impressed enough to be brought on board by manager Micky Adams.
The 24-year-old, who can play either on the wing or in attack, will go straight into Vale's squad for Saturday's home clash against Grimsby.
Adams told Vale's official website: "I have been impressed with Lewis during training and I'm pleased we have managed to secure him on a month's loan.
"He will bring some width to the squad, as well as a different option moving forward."
Haldane added: "Hopefully I can get some games now. It's been a frustrating time at Bristol Rovers and I haven't really had an opportunity in the last seven or eight months. Hopefully I can do the business, get a few matches and maybe stay for longer."