Mikuni carbs are fairly complex and the ones on the Virago are the worst I've seen. Someone took them apart at some point and had a couple of very small parts left over at the end. Idle mixture screws are normally a simple screw with a pointy end and a spring to hold it in place once you have (mis) adjusted it to your own personal preference.
I will note that my 84 Honda Sabre still has the anti-tamper covers from the factory installed on all 4 carbs and the engine runs fine. The Yamaha's screws are located deep down a hole where you might well not even notice them, then covered with a plug. They also include a washer and an O-ring along with the spring which must serve some fairly important purpose or they would not have been included. The O-ring and washer were the leftovers on both carbs.
The main jet needles have been through one model change and 2 revisions so I ordered 2 of the latest versions. Needles were ordered 6/23 from Omaha, and the o-rings 7/1 from Seattle. I can't wait to see which one gets here first.