At what stage (and what deliverables) would a VC/investorvalue the services of a market researcher for the purpose of validating investment?

I assume that you are referring to a professional market research firm that would validate your target market, correct? If so, to the extent you can get credible, independent validation of your target market, it will help with the credibility of your business plan. However, since you will want to focus on the 'addressable market' (how much of the total market really applies to your products and services) and, most importantly, whether or not real customers will actually buy your products/services, I would not spend a lot of money on an expensive research firm but I would get professional advice and help to make sure that you have good data. If you are at the stage to get serious VC interest, they will do a good deal of due diligence and research on the market segment(s) that you identify to make sure that you are right. Remember that the best validation is contracts with and sales to strategic partners and customers.