It's a disadvantage to have only one VS in the sense that most people on a given VS will not get a TC, so more VS increases the chances you will get a TC.Is it a disadvantage to only have one vacation scheme? Would having multiple vacation schemes make a candidate more appealing/stand out more especially when firms are deciding on who to convert to a TC post VS?
However, the firm is going to meet you for an entire week (or two), and will be able to judge you quite reliably based on your performance over this time, and isn't likely to factor in how attractive you appear to be to other firms into its decision-making about whether to offer you a TC.