Do I need a contract for each vendor?
The short answer, yes! Digging deeper, if your are going through a wedding planner whose preferred vendors you plan to use, you may be able to book the vendors through the planner. In this case, the planner acts as the contractor, however, there is usually an "up-charge" (10-20% added to the vendor's fee) for this service, which is not a problem for couples with an endless budget. For the other 99% of couples with limited budgets, it is an issue.
It has been my experience that vendors of quality (the best at what they do in the area) do not need to be associated to one particular planner. They are already busy....
Back to contracts: Professional vendors use a contract - plain and simple. A properly executed contract (listing the date/time/type of service/compensation/terms) and deposit is a binding document assuring a couple that the service will be provided.
A case in point: I spoke to a bride in early November about her wedding in March. I sent our demo materials (which included our booking terms) and in two days she responded to me (via email) that what we provide "fit her vision for the day" and that she would get back to me to firm things up. Upon hearing this, we put the date temporarily in our schedule. Weeks/Holidays passed without the client ever getting back to me...so we deleted the date.
During the first week of January I received an email from the above-mentioned bride with a list of songs to be played at her wedding in March. My response: "at this time we are not booked for your event" bride: "I thought that we were all set..."
At that point I re-checked the date with the rest of my ensemble...we were all still available. I sent a contract, which was promptly signed and returned, after which we were finally booked.
Fortunately the wedding in question was in March. If the wedding was in May - October the story would probably have ended differently...
The short, long, and only answer to the original question: Yes, you do need to sign a contract!