Even if they don’t get it back through sleeping, if the assumption is that some “normal” days pass, then a character should be indulging their virtue or vice during that time and so I would argue that even a Vampire or Sin-Eater should get some willpower back after a time skip. RAW if characters get back willpower through sleeping then a 10 day time skip should fully recharge anyone.
I also find that every character constantly running off to fulfill their Vice/Virtue can interrupt the flow of the game in a negative way - if the players know there’s a big fight coming and they’re low on willpower they’ll all suddenly want to run off and do their “good deed” to get their willpower back, but I think willpower recovery should be a more seamless part of the narrative.
Personally, I’m not very good at remembering to fulfill my character’s Vice/Virtue equivalent and neither are most of my players, so I tend to be very lenient when it comes to willpower recovery, and actually let my players regain free willpower at the start of every game session. In terms of managing the resource of willpower, it’s down to you how strict or lenient you want to be.
In the FAQ it’s explained that if a core book doesn’t say the characters recover willpower from sleeping then they are one of the exceptions (I know Vampires for example do not regain willpower through sleep). Not all supernaturals play by these rules though, and the general rule seems to be that undead characters don’t benefit from a night’s sleep the way mortals do. World of Darkness, a world very much like our own, save that the shadows hide very real monsters (though most people fool themselves into believing it isn’t so). Basically the default is for mortal characters to recover 1 willpower per night’s sleep. This has been addressed recently in the Player’s Guide to the Contagion Chronicle, which was just released to Kickstarter backers and contains a nice cross-splat FAQ.