While shaking would likely add some energy to the system, I do not know if it would add enough to change the temperature (and thus the pressure) noticeably.

The liquid/gas: butane. The pressure vessel: a disposable lighter. As the fuel runs low, so the pressure inside drops until you reach the point where the flame is very feeble. Out of habit or instinct, I tend to shake it for a few seconds, and try again. Frequently, the flame is stronger afterward, I got to wondering if this effect was my imagination or real.

The likelihood is that any significant increase in pressure is due to the warming affect of my hand holding the lighter. But try as hard as I might to not grip the lighter whilst shaking it, so as not to cause it to be warmed by body heat. Still, shaking it does seem to produce a noticeable increase in pressure. Hence the question.