10% paid and then vendor doesn't want to complete

If the 10% deposit is paid and the purchaser then doesn't go ahead, they lose their money.
What happens if the vendor doesn't want to finish the sale... Do they have to pay the would-be purchaser any recompense (on top of refunding their 10% deposit), or is it risk-free for them?

All depends on signed contracts.
If none, you get your money back, no more no less.
It works both ways.

Although its unlikely that you would have paid the 10% deposit without signing Contracts, they go hand-in-hand. So where does that leave the Purchaser if the Vendor pulls out? Does he sue under the Contract to make the Vendor complete?