I want to know whether, if a vector file is rejected due to a quality issue, intellectual property, or similar content, I can modify the file—for example, by deleting some parts or adding something new—and then upload the revised version again.

Sorry, by “twice,” I was including the first time. You should RE-submit only once, unless you make extremely drastic changes to the original

For inspiration on what to create, see Popular Subjects below:

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-photos/photo-submission-overview.html

Also read the 7-part Vector Requirements. These were updated last month. The new file size & artboard specs have increased as well.

https://helpx.adobe.com/stock/contributor/submit-your-content/submit-vectors/vector-submission-overview.html

Best advice: stop submitting icons.

You’re competing with too much similar content. It’s a losing battle at this point. 

https://stock.adobe.com/search?k=icons  160+ million results

Stock needs many other things besides icons. Find a niche that needs filling and create content to fill it.

Good luck.