Hello, I hope the following is of assistance.
This may well be 2 separate concerns. The vibration and nearly stalling is likely to be separate to the gear selection problem.
This engine is a little prone to misfires usually due to a faulty ignition system, this makes the engine run on 3 cylinders which causes the vibration and also the idle speed drops. sometimes the amber engine warning light comes on when this has happened. Check that the engine is running on all 4 cylinders, look for any sign of arcing on the side of the spark plugs, and if necessary have the coil output checked, it's not uncommon for the ignition coil or a HT lead to fail.
With regard to the gear selection and oil loss, check that it's not brake fluid that you're seeing as the clutch hydraulic system uses the brake reservoir for it's fluid, if the clutch slave cylinder fails brake fluid comes out between the gearbox and engine.
If it's definitely gear box oil then the source of the leak needs finding before the level does get too low and causes damage, the only place oil usually leaks from is a drive shaft seal.
If the clutch slave cylinder has failed then the gear box needs removing to replace it, it would be good practice to replace the clutch assembly at the same time if you do this.
Start with these points, let me know if you have further questions regarding this and I'll reply as soon as I'm able to.