Hi, Peter and the usual suspects are:
1. Improperly loaded motorcycle. Non-standard equipment on the front end such as heavy radio receivers extra lighting equipment or luggage tends to cause unstable handling.
2. Incorrect air suspension pressure.
3. Damaged tires or improper front-rear tire combination.
4. Irregular or peaked front tire tread wear.
5. Incorrect tire pressure.
6. The shock absorber is not functioning normally.
7. Loose wheel axle nuts. Torque to recommended specifications.
8. Excessive wheel hub bearing end play.
9. Improperly vehicle alignment.
10. Steering head bearings improperly adjusted.
11. Tire and wheel unbalanced.
12. Rims and tires are out-of-round or eccentric with the hub.
13. Rims and tires are out-of-true sideways.
14. Loose spokes.
15. Shock absorber improperly adjusted.
16. Worn engine stabilizer links.
17. Damaged rear engine isolation mounts.
18. Swing arm pivot shaft improperly tightened or assembled.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
VTX 1800 Handling Problems
http://www.triketalk.com/forum/archive/index.php/t-1065.html Honda VTX1800R Owner Manual
service manual, not owners. https://www.partsfish.com/page/oem-parts-for-honda
Honda VTX1800R Owner Manual