Being able to create multiple purchase orders with different vendors from an estimate is a great idea, @paulhelsby.
For now, you can choose specific or all items when you convert an estimate to purchase orders in QuickBooks Desktop.

While the said option isn't available in QuickBooks, I recommend visiting our blog sites to keep up with the recent news and product release. Here are the articles you can review to achieve this:
Also, the good thing is, you're able to run either the Open Purchase Order List or the Open Purchase Order Detail report to keep track of all the purchase orders you've created. Just go to the Reports menu, refer to the Expenses and vendors section under the Standard tab. Please see the screenshot below for visual guide:

Feel free to add a comment below if you have more questions regarding this or any QuickBooks concerns. I'll be here to help. Have a nice day.