Your entire order gets executed at your limit price, or better.
Should a better price be available than you originally indicated, the size of your order will stay as originally input, but you will be deleveraged if needed.
Alice sets a limit buy for 1 ETH at 1500, while it trades at 1600. For this she is willing to provide a 750 USD margin. A large sell order pushes the vAMM price below 1500, so the bot executes her order. Alice will end up with a position of 1 ETH at 2x leverage or less.
Bob sets a reduce-only limit sell for 100 Sushi at 18 USD. He already has a 100 Sushi long position and he only wants to take profit, hence the reduce only parameter. The bots will only execute the order at the given price or better.