I am holding a VTI position that has unrealized long term gain. I would like to replace this with SPY to use as option collateral (VTI options spread is bad). Is there a way to do so without incurring capital gains taxable event?
No
You could create a fake identity then fake your death leaving your VTI shares to yourself (the fake identity that is). Your inheritance will get a step up in basis so no taxes owed and you could use those proceeds for your SPY buy/write and live to fight another day. Foolproof.