I don't know the books so this may not be helpful, but some thoughts:
Yeah, I think what base system you're using and what sort of vibe you want it to have do matter a lot here: I'm guessing that in a sense it won't be horror per se in that your players will be the vampires (so it's not horr
ifying because, well, you're the thing people should be scared of). But it'd be worth seeing what's out there, I don't know how well the Vampire: The Masquerade style games handle historical stuff as IIRC they're very dug in to their specific vampiric setting and quite modern themed. But doing Savage Worlds, GURPS, or D&D might all work, I'd
possibly avoid D&D for this in that I'm not sure it's as well kitted out for pseudohistorical stuff especially when you get into the early modern period.
Second block of thoughts, my usual starting approach is to give a berth around a period I want to theme on and start working out what can be gently pillaged from other media, so for your case I guess anything set 1750-1830ish might be fair game. That means you can throw in bits from Fenimore Cooper, or from Victor Hugo, or Jane Austen, or Jules Verne, as random inspiration points. Les Miserables and The Hunchback of Notre Dame might be interesting inspirations to play with, you could do the runaway cart scene from Les Mis and give the players the choice of whether to use vampiric strengths to stop a disaster at risk of potential suspicion and discovery.
Third thing, revolutionary period France has a lot of politick and thinking about how that interfaces with vampiric-ness can give you sidequest ideas. Vampires have a sort of inherent aristocrat thing going on, even if they're vampires made from commoners. Their. That's really what makes the revolutions period so good for vampire settings, the idea of the landlord/aristocrat as a literal parasite but with an alluring nature is a key one. But how you interact with that is up to you: do people assume your vampires are aristocratic given penchants for hiding in the day and wearing super fancy clothes? What does that mean when people want to guillotine the aristocracy? That's not the only key division either, the Montagnard/Girondin thing would be really interesting with the Montagnards somewhat representing ferocity, even the more bestial side of vampires, being the mobs-and-inward-looking types who think that vampires' ability to kill should be directed against the elite, whereas your Girondin vampire would be much less outwardly predatory and would be busy building networks with vampires abroad, working with early chemists and designing orreries, imagining a world where they can get on with their painting, etc, possibly to the detriment of actually achieving their political goals.
So boiling that into possible sidequests:
- Have some political players, vampiric or not, who the players can interact with. Sidequests can then be very factional if the players want to get involved in politics. Possibly if the political players are not vampires, they have discovered something they can hold over the players, or are willing to protect them in exchange for the use of certain of their talents. This could lead to quests around assassination, delivering messages past guards, saving people from soldiers and the guillotine, hunting down people to have them guillotined, etc.
- Merchants also are a big deal at this point, we're in the French empire and there's a whole bunch of slavery going on, so particularly if they end up in any ports there's options for theft, freeing slaves, etc (honestly I think Bordeaux as home of the Girondins and France's major port might be as fun as Paris here).
- Early chemistry and science I think also have a lot of role to play, this is the period of frog galvanoscopes and phlogiston and Lavosier and so on. That could act in various ways, from a scientist wanting to capture or persuade a vampire to experiment on them, a vampiric scientist with a huge laboratory who equally might want to experiment on other vampires or humans, whether to find antidotes or poisons that work on vampires or finding ways to enhance certain gifts, etc.
- I'm sort of assuming someone will be hunting the vampires, I think you need that to provide pressure. Ideally an organisation so you can have a bunch of showdowns, and/or someone who the vampires actually don't want to kill for other reasons, so you need to kill off occasional thugs and minions, or lay false trails, or talk your way out of problems to avoid blowing your cover. So that's another big area of possible sidequests.
I think the biggest issue with vampire protagonists is determining what they
want. Because it's easy for vampires to lie low and get blood sometimes, that's kind of their whole deal. I think in the C18th the politics and sense of change mean everyone has to be wrapped up in that side of things, and that it will put them under pressure due to people hunting them or wanting to force them to take sides. But also you don't just want the whole campaign to be about them trying to survive: so in other words knowing your player characters is vital for good sidequest design, because otherwise you end up sitting there saying "well you could do X" and having players looking at it and saying "that's dangerous and I don't know why I'd want to". So ask your players at character creation to consider some obsessions, goals, interests, etc for their characters: if you have a painting obsessed vampire, say, they may be desperate to get their portrait painted by a famous artist, who has to be contacted, not informed of the vampires' nature, with vampire hunters kept off the scent, and the artist may also for example be a former royal portrait painter who the montagnards want to kill but the girondists don't to add further complications. So simple character quirks like that can build sidequests that players feel are much more relevant to their characters and goals.
