It's common that deposits will be required from most vendors. This not only protects the vendor, but also the client. In almost ALL cases, this deposit will be NON-REFUNDABLE. It is a necessity for vendors to keep this deposit, in the event of a cancellation as they have lost potential bookings because they had held your date for you as per the contract. Usually the deposit is 25%-50% of the contract total cost.
So let's say the unexpected actually happens, and you have to cancel the event for any reason. There is a way that you aren't COMPLETELY out your deposit. I usually suggest that prior to signing, you see if a clause can be added where in the event of a cancellation, the deposit can be applied to another event (not necessarily a wedding) within 1 year of the event. Keep in mind, the vendor may still not have your new event date available, and therefore still entitled to your deposit, but it's worth a shot rather than being out completely.
Any additions, changes, or verbal agreements that client and vendor are making should be written down and initialed by both parties. Too many times verbal agreements are made and as vendors get overwhelmed with the sea of other clients they have, it is forgotten what exceptions were talked about so long ago. In short...get it in writing!
Keep in mind, weddings are a "luxury expense" and luxury always comes with a price. Most of the vendors are priced to be competitive in their field already. I always feel that if a client is asking for a discount, they must be ready to receive "No" as an answer.
A vendor is in no obligation to offer a discount, and Clients should be respectful in their request. Instead of shooting off a low ball amount that could potentially offend the vendor and their professional quality of work, ask them what discounts they DO offer. It is much better to ask if there are any specials in the upcoming months they should wait for (such as booking in off season, or upcoming bridal fair specials), or some vendors will offer military discounts, etc.