Not all SVGs are created equal. A vector that looks perfect on a website might be a nightmare for a 3D engine.
Rule #1: Close Your Paths
For an object to be 3D printed, it must be "watertight". In 2D terms, this means every shape must be a closed loop.
If you drew a line but didn't connect the end back to the start, Vextrude doesn't know what is "inside" and what is "outside", so it cannot extrude it.
Beware of Self-Intersections
This is the most common error. If a path crosses over itself (like a figure-8 loop drawn as a single line), it creates mathematical ambiguity.
How to Fix:
Open your SVG in Illustrator or Inkscape and use the "Simplify" or "Union" tool to merge overlapping geometry into a clean single shape.
Strokes vs. Fills
3D engines extrude Fills (the color inside the shape), not Strokes (the outline).
If you have a logo that is made of thick lines, you must convert those strokes to paths (Object > Path > Outline Stroke) before uploading. Otherwise, the 3D converter will ignore them.