Vegeta and Piccolo get stomped. 17 said 18 wouldn't be able to handle all of them at once, not just the two Super Saiyans. Given how quickly she one destroyed Vegeta and Trunks, subbing out Trunks for a weaker Piccolo certainly won't help their chances.
Given how well Vegeta was keeping #18 at bay before his stamina faded, I think Piccolo could win with a well planned strategy. Something similar to what he and Goku did against Raditz, perhaps.
For reasons SSJ2 already mentioned, #18 still wins. Piccolo could prove some difficulty through teamwork mitigating Vegeta's stamina loss, but these two actually being able to work well together is a pretty big "if".
Trunks said 18 and Vegeta were equal, so they're equal. Forget Imperfect Cell and suppressed Kamiccolo, that's the only statement and it can be interpreted no other way ever. Eat my ass.