How to deal with a gap between vanity and wall

If you have access to the same tiles that you used, you could cut them to size (you may want to consider using a good cutter or wet saw). That way you can caulk the space in between the tile and the acrylic shower base. If you don't have any extra tiles or can't find any more of the same type, you could find a nice complementary looking small tile and install that instead.

If your floors have their seams lined-up like the diagram (very wrong), it looks quite bad already so adding a bunch of scraps on the end won't hurt. The best solution is to literally start over & use as many of the old pieces in the manufacturer's instructions followed new & proper floors...that have at least 3 rows laid as a dry fit so you don't have any pieces shorter than 30cm.

Each row of planks has its butt seams offset from the next row & at least the same for a third row before you repeat. One main reason for this is exactly your situation here. Be careful & conscious with your measurements & cut 1 board at a time...do I leave the line, cut-out the line or split the line.

With varied lengths meeting the wall you can custom cut & even scribe each piece's end for a flawless fit or common very minor manufacturer's gap. Also, any ridges, popped nail heads, splintered subfloor faces, heavy paint or glue deposits, etc. should all be flattened, leveled, sanded, chiseled & removed prior to installing the new floor.