Vegetable oil is certainly cheaper than chainsaw oil, and the first thing that is noticeable is the viscosity, as vegetable oil is a lot thinner than chainsaw oil.
I have only been running my chainsaws on vegetable oil for about 18 months, so theoretically it is still early days, and have not noticed any chain or bar wear or build up of "gunk" (sawdust & oil).
However I have read mixed reports from professional Arborists, who use their chainsaws for lengthy periods of time day in and day out, and really put their equipment through their paces.
Some Arborists mention wear not to the chain, but to the bar, and also the resinous build up of oil and sawdust which occurs round the clutch drum, oil pump, chain brake etc. However, there are also reports from Arborists who have been using vegetable oil in their chainsaws for years without any issues, so it is really a mixed bag.
Another issue which could be carried over into a further topic would be:
" would you really know the difference if vegetable oil or conventional chainsaw oil was used to cut up your smoking wood?"
I have an old chainsaw that has not been used for a few years which still has conventional chainsaw oil in the tank, I am quite happy to provide a mixture of wood that has been cut using both saws to see if you can tell the difference.