For mountain landscapes, your 18-200mm should already work well. Landscapes are usually shot stopped down, often around f/8 to f/11, to get plenty of depth of field, so a fast maximum aperture like f/1.4 usually isn’t a big advantage.

On a crop body, the wide end of your zoom is likely the most useful for broad mountain scenes. A 50mm prime would generally be too narrow for many classic landscape views, though longer focal lengths can still be excellent for isolating distant peaks and details.

If your concern is sunrise/sunset light, a tripod will usually help more than buying a fast prime. Since mountains don’t move, you can use a slower shutter speed and keep the aperture stopped down for better depth of field. VR also helps with camera shake when shooting handheld, but a tripod is the better solution in low light.

If you need a wider view than 18mm provides, you can also shoot overlapping frames and stitch them into a panorama later.

So: no, you probably don’t need a fast prime for this. Keep using your zoom, and consider investing in a tripod or small support if you don’t already have one.