It's easy for Vash to melt into that kiss. When he's with Cyril it's the only time he finally feels like he's wear he belongs.

At first the kisses are light, gentle, and slow. Vash letting them linger and enjoying this moment with Cyril after wanting some time alone with him for so long.

Then that unnatural feeling burning desire within him begins to creep up. The kisses gradually becoming more hungry and desperate.