How do VC firms support marketing for the startups they invest in?

Effort put in by VC firms toward the marketing of their portfolio varies from firm to firm. In my experience it spans the spectrum between nothing and somewhat less than you've described. The expectation/average/mode behavior in the firms I have worked with was "we have a high-quality PR firm that we work with (and recommend to our portfolio companies) who understands the space, the relevant publications, and will work with you on cost because of our relationship". Marketing does not equal PR, but this is what I have seen "offered". For anything more detailed and company/industry specific than that only a tiny fraction of VCs have in-house staff that will come in and get hands-on for things like direct user acquisition or email campaigns or ad-placement or social strategy. More likely, a VC might line up a particularly marketing savvy portfolio company with one in need to share tricks and tips across the portfolio but that would not be more casual than formal exercise.

Definitely the answer here is "it depends on the deal cut in the first place" but I can tell you from a friend's startup that I know, they got funding from a huge company's VC fund (not this, but similar to AOL's venture cap incubator) and offered them the services of their in-house marketing research team to help figure out strategy, find resources etc. This was not free but billed back to the startup at a discount that got less steep every year. The in-house firm also offered startup consulting services to startups who aren't even funded by the fund so it's a service that is out there.

I work for a VC funded start-up, in London, UK. We are basically a self-sufficient company and are expected to use the funding to arrange our own marketing, PR, advertising etc. In my experience, the extra value that you might get from VC firms is not so much any service they provide than the networking and introduction opportunities to other businesses that you can work with in your industry. In fact, that's the secret of start-up culture really, that the success of any early business is 90% networking, 9% product development, 1% great idea. There are business incubators, business coaches, etc. who will provide these on marketing, PR, communications etc. support, or consultancy on these services, but certainly for us we had no expectations of these being provided by VC firms.