Do not use washing up liquid - it is full of salt and phosphate so will rot your metalwork.
VW used talc in the factory and that's what I've always used. It works MUCH better than any liquid because unlike liquids it can't be squeezed out of the joints. Use thin pvc coated washing line, start at the top or bottom (it doesn't matter ) but don't press too hard on the glass to force it into the aperture - that will simply trap the cord. You are aiming to use the cord to JUST pull the rubber lip over the body tin so you pull over about a foot or so, then push the glass in towards the body a bit, then pull over another length, then push a bit etc etc. It's always worth comparing the new glass with what you have taken out - I've seen replacement windscreens a full 8mm larger than the original :shock: If you are supplied with an oversize glass, take it back and find another supplier.
Do not under any circumstances use silicone compound, either to seal or lubricate the glass - you will have fish eye nightmares if ever you need to repaint the van.