Mk3 vento (u.s. jetta) 1996

[Waxel]

Just bought a 1996 vento.  Believe it shares much with the golf mk3. Has 150k miles but only 1 owner and full service every 10k since new (new timing belt at 145k).
Couple of questions (sure there'll be more) - first can anyone tell me what this pipe on left of engine is going from bottom to top of engine (pictured)? I noticed it was disconnected at the top and stream was coming out.
Second,  it has a small coolant leak which I think is coming from the thermostat/coolant flange under the distributor cap. Anyone have experience of fixing this? Thanks