in my experience they will provide a terms sheet to you and they will ask you for what you think your valuation is. If they think it's too high they will push back or walk. yes, it is possible to blow it by setting your expectations too high. However, remember the adage "the only thing a VC hates more than a bad investment is missing out on a good one" - if you have a compelling team, product, traction, etc. and you are realistic about terms and performance then you will be fine. understand what makes them tick - they want you to be successful. Yes, they are in this to make money for themselves and for their investors, but that doesn't happen if you are beat to a pulp and de-motivated. There needs to be a fine line between terms that make sure your team has a sense of urgency and commitment and big rewards for big performance, but not so draconian and controlling that they dread coming to work. There are certainly deals that get done where the terms are too good for the investors and everyone looses.
Given that you are looking at a convertible note then valuation is somewhat a moot point and their terms will focus more on discount and control-type provisions such as anti-dilution, liquidation preferences, board seat, etc. but i have not actually done a convertible note. You should be able to get a good feel for what valuation other companies like your's (your stage, team, location, etc.) are getting - it is surprisingly "standard". I don't think you will have to push to get them to lead the discussion on terms. If this is a seed-stage VC they will understand the need to have terms that are palatable for the follow-on investors so they shouldn't be too heavy handed or they won't be able to attract A round investors. That being said they won't be terms that are easy to swallow either. Understand that with a convertible note you are on a strict time clock - if the conversion date arrives and you don't have your follow-on deal done the discussions to renegotiate the terms of your note will make you sweat. good luck.