After banging my head again at the weekend
I finally replaced the 16 year old gas struts on the Vito's tailgate today.
I'd been putting it off for ages but it turned out to be a really quick & easy (and cheap) job. The most fiddly thing was finding something to hold the (large and heavy) tailgate fully open while I did it (I used a ladder in the end). After that it took a minute or so each side - they are just held by a C clip at each end, which was easy to lift with a screwdriver. The new struts then simply snap on. I bought a pair of mid-range struts (claimed to be from an OEM manufacturer) for £15 delivered on eBay - they looked good, fitted perfectly and work fine. Checked Euro Car Parts first and struts there were £25-£50 each.