Hear is how to change this and I would call this a bug in vbulletin but hey they can do what they want. See the issue is they set in there default style the popup background to white but then in 4.18 they are also setting the styevar that I posted above links to white also. This seems like a mistake to me.
You can go to your vb
style manager in vbulletin admin panel and edit templates go down to css templates and edit popup.css and find the part I meantion change the color to what I note as right now they have it set to the same color as the blockhead link color which is white on top of blue which is fine. Do not know why they would set white on white.