Hi there, @NHSMP.

As my colleague stated earlier, this thread is a mix of Online and Desktop. So, I don't want to assume which product you are using so. I'm going to apply both help facts in this response. You can only apply one vendor to one check in, both QuickBooks Product. I'm going to list how to do multiple at a one time with separate checks.

QBO Online:

1. Go to the +New button.
2. Now let's select Print Checks.
3. Load the printer with your checks.
4. Select the bank account that has the checks your write that need to be printed.
5. Starting check no. field, enter the check number of the check in the printer. The number is found to  the right of the account number at the bottom of the check.
6. Then choose Preview and Print. If everything is correct, select Print.
7. Now hit OK and select Done.

QB Desktop:

1. Go to the Banking, then select Write Checks.
2. Click on the Print Later box next to the Print icon for each check you want to print, then close the Write Checks window.
3. Afterward, select File, then choose Print Forms. Choose Checks or Paycheck, as appropriate.
4. Now click on your bank account and select the checks you want to print. To do this, place checkmarks next to them.
5. Enter the first check number, then choose Ok.
6. Here, you're going to pick Check Style (Wallet, Voucher, or Voucher), then be sure that the correct printer name is showing. 
7. Now select Print.

Now you're able to print your checks with multiple vendors. To know more about this, just visit Print Checks for Vendors. I'm glad I was able to lend a helping hand to you today. If you have any further questions, please don't hesitate to reach back out to me. Happy Tuesday!