When you convert VMDK file to thin will it take into account that partition is been deleted ? because actual data is still on HDD, Windows just marks it has deleted but does not delete actual data.
You may need use some software like sdelete to write zero in all blocks that previously has data, this way VMware will recognize this like free space.
Will it create new 50 GB VMDK (thin) or will it create 1.5 TB VMDK file (thin) ?
It depends, it will only decrease in size if VMware detect true free space (with zeroed blocks).
Take a look: https://www.virten.net/2014/11/howto-shrink-a-thin-provisioned-virtual-disk-vmdk/