The trip on the N'EX is relatively short, but I have seen the "trolley dollies" come through with beverages and snacks. At least I know they had them a few years ago; I don't know if they still do. I've never seen on-board vendors on the Skyliner, though.

However, if you really need some sustenance before boarding the train to Tokyo, there are a Starbucks, a kiosk selling bottled drinks, packaged snacks, and reading material; and a couple of Japan's ubiquitous beverage vending machines by the entrance to the platforms.