When i copy and paste a vector into another document, it doesn't paste everything

There could be several reasons for this, mostly rooted in the nature of the content you're attempting to copy. Since you're new to this, I'll try not to assume anything.

I am trying to copy and paste a vector image from the original AI file

If you know the difference between vector graphics and raster images, you might come to realize that calling something a "vector image" is a contradiction, but more importantly, you should understand that an Illustrator file can contain both vector and raster based content. Also due to the way you phrased it, I suspect "the original AI file" may contain artwork you didn't originally create. For that reason, it's fair to imagine you may be unaware of its construct in some respects, and the same can be said for me and anyone else reading your post.

If you select all (Ctrl/Cmd+A) and copy (Ctrl/Cmd+C), and get incomplete results when you subsequently paste (Ctrl/Cmd+V), it's probably because everything wasn't actually selected. Most likely, that's due to layers or objects which are locked. To help get to the bottom of it, the best next step would be to inspect the Layers panel. If you could make a screenshot of your selected artwork, with the Layers panel also in view, it will be much easier to offer specific guidance.

SIDE NOTE:

. . . so I can use within a clipping mask to create a logo.

This may be a misguided objective. If the content you're copy/pasting is 100% vectors, you really shouldn't need to mask it, and, a logo graphic destined for deployment as a brand mark is no place for a clipping mask.

You should check that all items are unlocked and all layers are unlocked before copying.

Do so in the layers panel.

If it still doesn't work, please show screenshots.