Paying your vendors for products or services can be recorded easily in QuickBooks Online. When you receive a bill from a supplier, you can enter this bill into QuickBooks and record a payment using the Pay Bills page.
It sounds like you're trying to pay your vendors through an online deposit or QuickBooks Payments. At this time, QuickBooks Payments can only be used to receive funds from your customers, and can't be used to process payments you're making to others. I'd be happy to pass along your feedback about this to the engineering team. We strive to include as many customer-suggested features in our updates as possible.
As a workaround option, perhaps your vendors would be comfortable receiving an E-Transfer from you, which moves funds directly from your bank account to theirs. This is something you can discuss with them, and fees may depend on your Financial Institution. You're able to record the payment in QuickBooks as an E-Transfer if that's what you decide to do. The steps on how to do that are listed in the article I linked above.
I hope this helps. Let me know if you're looking for more information on this and I'd be happy to help.