@TheOriginalFrench's advice is spot on. It's not just about clothing, it's about self-care and investing in yourself too.
Vinted has numerous pitfalls, so unless you know what you're looking for and/or buying, and the seller supplies enough pictures and information including measurements, it's not quite the solution. And while some like @Keleshey have found amazing finds in charity shops, I've rarely, if ever, found anything, and believe me, I've spent a lot of time looking.
Regrettably, I don't think looking good is as simple as buying a few t-shirts, some striped tops and a great pair of jeans, (although having a selection of the latter is essential if you like wearing jeans). Not everyone suits or even needs these items. The idea of a capsule wardrobe is great in theory but in practise, the oft cited staples aren't things that everyone wants to wear/suits their needs. It's better to use the idea of a capsule wardrobe as a guide to building your own version of it rather than following the guidelines religiously.
Before so much as buying anything, sit down and think properly about what clothes you actually like. Whose style do you admire? Sometimes, once you figure out the latter, the former is easier to emulate. Pinterest as good for this, but so is Instagram. Instead of looking at influencers, look at brands and then look at their tagged pages to see how people are wearing their clothes. It's a shame Matchesfashion is no longer trading because their styling was great. Even so, e-tailers like Mytheresa, Net-a-Porter and Farfetch can be used in a similar way. Additionally, if you find pictures online of outfits you like, use google lens to identify where the clothes come from. Then you can find more purse friendly alternatives.
'And, bearing in mind I have a limited budget, what items should I buy so that I can get maximum use in terms of mixing them up, making them look different etc.'
The key, I think, is spending your money well. Look for quality rather than trends, and it will take a while to build a decent wardrobe. Accessories - bags, belts. jewellery and shoes - will maximise the potential of the individual pieces and making outfits look different.
Happy shopping OP