From what I know, the vaccine-induced thrombotic (blood clots) thrombocytopenia (low platelets), i.e., VITT, is not caused by spike proteins but rather PF4 antibodies.

Only the adenoviral-vectored DNA-based vaccines are known to cause VITT. And scientists believe that the unwanted interactions between adenovirus tech/DNA with platelets/PF4 antibodies is the cause of VITT.

The vaccines use anchored spike proteins that don't bind to the ACE2 receptor, so I doubt there's any theoretical basis for how vaccine-derived spike proteins could cause blood clots. If vaccine-derived spike proteins can cause blood clots, we would see blood clotting incidents with all the current spike protein-based vaccines, not just the adenoviral DNA ones. I hope that helps, but let me know if anything is unclear.