First, you will probably want to only use a single login form instead of one for each different user. Forms authentication can only send un-authenticated users to a single form, so it would be very confusing if a vendor tried to access a protected resource and was redirected to the Admin login form.
Next, you will probably need to setup Forms authentication to use roles, and define Access Rules for each directory that will keep users from un-authorized roles from accessing items in specific folders. Assuming that you have the following three roles for your application:
Admin
Customer
Vendors
you can then Access Rules as follows. In the Admin folder, create a new web.config file. Add something similar to the following to it:
<authorization>
<deny users="?"/>
<allow roles="Admin"/>
<deny users="*"/>
</authorization>
This ensure that unauthenticated users cannot access the Admin folder (<deny users="?"/>), that anyone in the Admin role can access the folder, and that everyone else cannot access the folder. (You may have to check into the syntax of this, I'm not sure if it's perfect). You will have to setup a web.config file for each folder you want to protect. Make sure that you allow un-authenticated users to access the directory in which your login form is located or else un-authenticated users will have no access to the login form, which they need to become authenticated.
So now the question is, how do you define roles, etc. You can define them in the web.config, or you can define them in a database and write some code to load them into the authentication ticket (you could do a lot of other things two, but those are probably the most popular). Here's some links on how to go about doing that:
http://www.xoc.net/works/tips/forms-authentication.asp
http://aspnet.4guysfromrolla.com/articles/082703-1.2.aspx
If you google for "asp.net forms authentication with roles" you should come up with a wealth of sites that can help you out.