non payment of tax by vendor

if the seller does not file returns, the purchaser can't claim ITC for that particular transaction and it should be reversed in his returns.

But that would mean that we the tax twice. Once to the vendor and than to govt. by reversing itc

yes Vinod ji is correct. if vendor is not registered under g gst though he is liable for the same and if he collects gst but not deposit the same to govt then the same is offence and penalty action will be taken on him but along with that customer also can't claim the input credit of the tax paid by him and have to reverse the input credit if already claimed by him in his return.

But what if the vendor is registered but does not deposit the tax

yes naveenji it means customer has to suffer when that is not his false. you can go into litigation or if you are a potentials customer then vendor would also not like to loose bis customer just for the gst credit.

In my view, you can check the status of your seller whether they are filing returns punctually or not. if they are punctual, then you can buy the goods from those dealers without worrying about the losing input tax paid on the goods.
In gst portal you can search the status of the dealer with their gstin number.