"Where do you buy eggs?"
Translation:Vy kupujete vajíčka kde?
"Kde kupujete vajíčka?" is actually the most standard/neutral word order.
The sentence in this exercise would most likely be used as a follow up to a statement like: "Já kupuju vajíčka na tržnici." (As for me, I buy eggs at the marketplace.) - "Vy kupujete vajíčka kde?" (And YOU, you buy eggs WHERE?)
Also note that "vajíčka" (sg. vajíčko) is a diminutive form of "vejce" (sg. also vejce). They are used pretty much interchangeably though.
Most of the exercises come in pairs. First a Czech sentence os selected and translated in English. The English translation is usually the most direct one, but if it would lead to undesirable translations to Czech, another one may be selected This English translation then serves as a reverse exercise snd the original Czech sentence is always the main translation.
Some exercises are Czech -> English only.
"Vy" is not needed. The problem is with the verb at the end of the sentence, which is the position where it's stressed. Using your word order, you're asking where you BUY eggs as opposed to doing something else with them. Please check the earlier comments for more info.
"Kady" is a dialectal (non-standard) variation of "kudy": https://www.reflex.cz/galerie/zajimavosti/98386/kde-jedete-hentam-jak-nam-nareci-muze-zamotat-hlavu-i-pri-uplne-beznych-otazkach?foto=2
But "kudy" does not mean kde/where. It means (through) which way. You can't use that in this question.
"Nakupovat" can be used intransitively (without an object) and then it means "to shop", e.g.: Jdeme nakupovat. - We are going shopping. Or: Musím nakoupit. - I have to do the shopping.
When used with an object, they both mean "to buy" and are mostly interchangeable, the shorter "kupovat" is preferred in normal circumstances, as the prefix "na-" is redundant.