There have been only two or three occasions in my career when I've needed to create a curvy vector line for a design in Photoshop. Each time it took me much longer than I anticipated. I'm not sure why, but what seemed like obvious approaches before always turned out to simply not work as hour upon hour went by.
I finally found a way! The best method I've found so far is to use the Line Segment Tool in Illustrator to create a straight line. Then use Filter > Distort > Zig Zag. Be sure to select "smooth" for the line. Be careful not to use the ZigZag inside of the Effects dropdown but instead confirm you are in the Filter dropdown before applying the distortion.
You should be able to copy/paste into Photoshop as a smart object now. I'm glad this has turned out to help so many other folks!
J. G., Owner GoForChange.com