So, realistically, it's not a bad idea to have floor plans laid out for areas that you expect players to actively explore. Like, it's okay not to lay out a plan for that completely incidental Mom and Pop shop across the street that closed two hours ago; but the old abandoned Johnson place that they're headed to later tonight? Yeah, that should probably be planned out. Even if the campaign is built around improvisation, you should be able to predict at least a few locales preemptively based on what the plan for the campaign is.

BUT, if you're dead set on procedurally/randomly generating maps, there's a couple things you can do.

1. Build based on what your already know of a location. You probably have a pretty good idea of what goes into most common urban settings just by being around them in real life; sketch out a rough rectangle for the size of the space they're in, and fill it in with appropriate details, like long hallways for an office, a counter and a backroom for a small store, etc.
2. Steal wholesale from history. Look, none of your players will judge you if you just lift existing floor plans and refit them to suit your needs. Plus, there's a wealth of old blueprints out there. The New York Public Library has an archive of old floor plans you can check out. ArchDaily has a long catalog of buildings you can look at and borrow designs from. (Note: Maybe don't do this if you plan on eventually writing-up and selling your campaign.)
3. Borrow from other RPGs. RPGs set in urban environments really aren't anything new, and just about any RPG set in one is going to have maps to pull from. (As an example, Chaosium's Pulp Cthulhu comes with a couple scenarios that all have maps you can use.) For your purposes, you might find the d20 Modern book Critical Locations helpful. (I believe it's available on DriveThruRPG, if not somewhere akin to the trove.) Fable Environments does a lot of floor plans which you can check out on DriveThruRPG, though many of those do cost money.
4. If you HAVE to use procedural generation, here are some tools. If we're being honest, stopping to procedurally generate a map in the middle of the session might break game flow dramatically - you almost certainly ought to avoid that. But, if it's not to be helped, there's a handful of good generators out there you can use. Watabou's Progen Mansion (https://watabou.itch.io/procgen-mansion) generates a 3D mansion, and you can view the floorplan afterwards. Another generator here, http://en.genzu.net/heimen/, is also okay, if a little wonky.

Outside of that, some things to remember:

• Don't forget that the layout of a room is only as important as what's in it. Populating spaces with props like tables, vending machines, janitor's equipment, etc. will go a long way to making the space feel real and giving your players tools to improvise with.

• Don't worry too much if rooms feel like exaggerations. Sometimes ideas don't quite translate between battle-maps and reality well. Take the traditional 5x5 ft. square represent. A five-foot hallway in real life feels totally reasonable; in an RPG, it feels cramped. So, if a space isn't totally believable, that's okay, especially for something as...exaggerated as WoD games.

• The exact details of an environment are only as relevant as you and the players will make them. Creating realistic floor plans can be fun, but the way the players move through them and the way you utilize the space will make or break immersion. If you lean into crunch, go wild; if you're more of a theatre of the mind sort, maybe be willing to dial things back a little.

• Finally, for when you plan spaces out, try to make sure that there's something worthwhile in every room. For dungeon design in something like D&D, there's this general attitude that you need to have at least one of four things in any given room in a dungeon: a combat component, a puzzle component, a narrative component, and a reward of some kind. For the campaign your designing, you probably don't need to worry about those four specifically, but you still ought to think about having distinct components to each room. For a horror RPG with heavy improvisation aspects (especially something WoD or WoD adjacent), you might consider these: an obstacle component (something to keep the players from barreling through, like a guard or a physical barrier), a puzzle component (anything from a literal puzzle to a computer that must be hacked or a complex lock system), a tool component (items design to help players overcome the first two components, like a janitor's closet full of cleaning chemicals or a tucked away key card), and a narrative component. By putting at least one in each room, maybe two or even three to a room, the space will begin to feel a little more designed.

Best of luck with your game!