Montreux to Paris, scenic ? I'd say yes, but from a photographer's point of view I wouldn' expect too much. TGV stands for "Train à grande vitesse" (high velocity train). Between Macon and Paris, that would be 300 km/hr, I have no photographic experience under such conditions. Just choose the one that best suits your programme.
Yes, you can go from Montreux to Gruyères in 1:15 hr, and from Gruyères to Broc, via Bulle, in 20 min. This site has a page where you can check the timetables, including between Switzerland and France. There, you can also check the train timetables from Wengen to Montreux, or any itinerary, including public busses (postal coaches and urban bus lines, cable cars, lake ships etc.).
Chalets for rent are usually available for weekly rents, Saturday to Saturday. But it is worthwhile to negotiate a shorter stay. Landlords are getting flexible these days. Otherwise, there ae hotels and inns ("pensions"). Good budget places are Youth Hostels (Hostelling International). They usually have double rooms and family rooms. You absolutely don't have to be a "youth" to stay there.
You can have vegetarian meals everywhere. I understand you do eat cheese. In the French part fondue and raclette are the traditional dishes. No meat in there. In the German part the mountaineer's macaroni is a traditional dish (Aelpler Maggronen), or Roesti (roast potatoes). You can get all these, plus "modern" vegetarian dishes, in all parts of the country. In the Italian part, a traditional vegetarian dish is pizzocheri (check Wikipedia). There is a vegetarian dish on the card of the dining cars in the railways. Don't worry. And enjoy a good Müesli for breakfast. Switzerland is the home of Müesli.
i don't think you'll hike on the Jungfrau. The Jungfraujoch is a train station with souvenir shops, restaurants, ice-carved statues (underground in the glacier), the make-your-own-candy place, and an outdoor observation terrace, surrounded by cliffs. The Jungfrau, along with the Eiger and the Moench, is an alpine peak, for experienced climbers with professional guides. On Righi you can hike, yes, it's a "cow mountain". I don't know Titlis.