#when you're a pathetic villain man a 'let me prove it's not poison' kiss is a hell of a lot easier to offer

[fleet-off]

This isn't a literal "let me prove that this medicine isn't poison" kiss for a whole host of reasons. The fact that putting the capsule between your lips for a few seconds says nothing about the capsule's poison content is actually the least of these, because the truth is that they both know Pete isn't scared the pill is poison.

Pete just told Vegas to let him die. Pete's not afraid that the pill will kill him, he's afraid it'll heal him and let Vegas continue using him as a plaything interminably.

Vegas isn't really saying "This pill isn't poison." He's saying "This healing isn't a trap meant to harm you." When he kisses Pete without a hint of taking or consumption or control, he's promising a change. And unlike the presence of literal poison, that is something you can demonstrate with a kiss.