One of the topics I often find myself advising clients on is what to do with an unresponsive vendor. Sometimes it's the venue or the catering company—kind of a big deal! Here's the advice I give them.
First, it is not at all unreasonable for you to expect a response within 48 hours of your last communication, whether it was via phone or email. Even if the response is nothing more than, "I got your email, and I'll get back to you soon." You have the right to expect prompt replies.
It is objectively unreasonable for a vendor to take a week (or more!) to respond to you without any sort of follow-up. Sure, everyone is busy, but ignoring clients is just not an option! So what should you do if you have a non-responsive vendor?
Don't keep waiting for them. If more than a couple days have passed and you haven't heard back, email again. You don't have to be mean about it; a simple, "Hey, I wanted to make sure you saw my email about such-and-such. Please let me know." will suffice.
If you don't get a response to your second email, call. Sometimes people have a hard time forcing themselves to type out an email, especially if it might be complicated to explain. If you call, you might just catch them and be able to get an answer right away.
If you're still having trouble, and you know who their supervisor is (if they even have one), involve that person. No one who owns or manages a business wants to have disappointed (or worse, angry) clients. If they find out that one of their employees is non-responsive, they'll want to do something about it.
Once you do finally hear back from your non-responsive vendor, you should make clear your expectations for response times. Tell them, "When I contact you, I need to hear back from you within x number of days. If you can't commit to that, I'll have to find someone who can." (Of course, finding another vendor only works if your deposit is refundable, which is not always the case.)
Don't worry about appearing "difficult" or "high-maintenance." YOU ARE THE CLIENT. The vendor should do anything within reason to keep you happy, and expecting replies within one or two days is reasonable. (Now, if you always expect replies within one or two hours, that would be unreasonable.)
If you're just beginning your vendor search and you're having a hard time getting responses from a particular vendor, let me save you a lot of time and trouble: don't hire that vendor! If you've already hired the vendor and you're having a hard time getting responses, cut your losses if you can easily do so. For instance, if your wedding is still six or more months away (so you have time to find another vendor) and your deposit is refundable. Always trust your instincts.