First, most Spanish speakers very much enjoy communicating with Spanish speakers from other countries or regions. One can enjoy the feeling of speaking slightly differently but still understanding each other. Also, finding differences in, for example, the names of foods, is fun.
It will be a little bit less confusing for the people you communicate with if you are able to pick one version, and learn that version consistently.
Which one you pick doesn't really matter. So, if you find a book at your second-hand bookstore that appeals to you (the presentation, the illustrations, the approach, the smell of the glue, whatever), then go with that. If you find a conversation partner who is from Country X, Region Y, then this (X, Y) combination would be a natural choice. If you know that you are going to do some field work next year in a particular country, then choose that country, to make your transition smoother when you arrive.
Note, I think variant would be a better term than dialect.
- Which dialect will be understood by the most people in the US? (Which is the most "generic")?
Probably Mexico in most places, but I imagine in New York City it might be Puerto Rico or Dominican Republic.
- Which dialect, if spoken, carries the least connotation (e.g., is not considered pretentious or slow/uneducated by the broader Spanish-speaking community)?
I don't know any versions that might be considered slow/uneducated. As for pretentious, you will only come across pas retentious if you put down others for the way they speak, or put on airs, claiming that your way of speaking is the only good way.
- Which dialect do sign makers generally use in the US?
Many publicly available signs (e.g. in hospitals) that show phrases translated from English to Spanish are full of mistakes. That, in my opinion, is the primary unifying thread. (Remember, I said many Spanish speakers are almost too tolerant of mistakes?)
- Will I be okay with a Spaniard if I fail to use vosotros?
- Will I be okay with Latinamerican Spanish speakers if I use vosotros?
In general, in the U.S., I would recommend that you not bother learning to conjugate for vosotros. However, if it gives you some sort of aesthetic or sensory pleasure, you could use vosotros in a quirky, personal way. There are weirder characteristics or customs one could have.
Important: If you find two or three learning sources that are particularly confusing in their differences, do post specific questions on this site. General remarks such as what I tried to give you are necessarily limited in usefulness.