Vexy Lines vs Vexy Linestra plug-in? Do they both do the same thing?

Vexy Lines is a standalone app. You can bring in your image, and then you can stack layers, mask out certain portions of the images and apply different vector patterns to them, and each of the vector patterns (fills) in Vexy Lines has a powerful set of options. For example, you can choose any pattern like linear, wave, radial etc. and then you can decide when (at which darkness) the strokes switch between solid strokes and dashed or dotted. You can make the strokes all dashed or all dotted. And you can manually draw guidance paths and have the Handmade fill fill the space between them. 

Vexy Linestra for Illustrator comes with a simpler, smaller set of vector patterns, with just the essential parameters like thickness, interval, angle, and a few more. However, Vexy Linestra works as a Live Effect in the Appearence panel of Illustrator. You can use Illustrator masking, you can use vector paths or bitmaps as source, you can then apply various built-in Illustrator filters — and THEN you apply the Vextra patterns. And then, again, you can apply additional patterns on top of the Vextra patterns. https://vexy.art/linestra/ shows what you can do. 

In short: Vextra is great if you want to mix some vector halftones or line patterns inside an Illustrator composition like a poster, and you want immediate results and flexibility of Illustrator. Lines is great if you want to spend more time to produce a more intrinsic complex multi-pattern illustration like those in https://vexygram.com/ — which you can then of course copy/export as PDF and place in an app like Illustrator to continue.