I would add the contact information in the order confirmation email (Settings > Notifications). You can use Liquid code to include different email address for different vendors.
Another option, would be to create a custom page with a dropdown where the user selects the vendor, then fills out a contact form and the form sends to the email address associated with that vendor.
If you get stuck and need to hire a professional, feel free to contact my team at speedboostr.com/contact. We do custom work like this all the time.
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Shopify Analyzer (first performance analysis tool for Shopify sites, free for the Shopify community).
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Order Automator (auto tag orders and customers + auto fulfill orders + email notifications).
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Tip Jar (add a tip button to your store).
