There can be a time lag - so much depends on the parties' particular circumstances.
There are a number of things you can do to safeguard your situation - if you feel threatened.
Sign contracts but make it a condition that Vendors sign contracts within X days/ weeks. Or if contracts are not signed within a certain period, advise vendors that you are pulling out.
Even in todays climate, when you would have thought that Vendors would be desperate to find and keep a purchaser, it can be the case that Vendors still are trying to tie in a sale and purchase which can be quite tricky. This might explain a Vendors reluctance to sign contracts.
mf