You're right, in plural form the translation could be "Guardo buenos recuerdos vuestros" or "Guardo buenos recuerdos de vosotros", both are ok. In singular form it can be "Guardo buenos recuerdos tuyos" or "Guardo buenos recuerdos de ti"

La mejor traducción sería: "Guardo buenos recuerdos de ustedes" (Latin América) or "Guardo buenos recuerdos de vosotros" (Spain) . https://www.italki.com/teacher/6042164?ref=6042164

Lo más correcto sería que fuera: "Guardo buenos recuerdos de ustedes"

FOnd doesn't mean good exactly, it's normally used for something u feel passionate about, and if u just translate this word to "bueno" it doesn't have the full meaning, for a more accurate translation i would use "entrañable/s": "Guardo entrañables recuerdos vuestros" or "Guardo entrañables recuerdos de vosotros" but, to be honest i would say that this way of expressing this idea is not very much used in everyday spanish, we just say good, yy, that's the naked truth. As many languages do, we also use 1 or 2 words to express something instead of all the synonyms every tongue has. In addition to what i wrote before, i must clarify that between Latin American spanish and the one spoken in Spain, (español peninsular) pronouns change, so instead of vosotros(Spain) we use ustedes(LA), therefore this sentence is gonna suffer some little changes: Guardo buenos recuerdos vuestros, Guardo buenos recuerdos de vosotros Guardo buenos recuerdos de ustedes, Guardo buenos recuerdos suyos. To sum up, if u want to be accurate in what u are expressing i would use "entrañable" whereas when it comes to using the language as a native would do just say "bueno".

Sugeriría un adjetivo que enfatice más el buen recuerdo, para hacerlo equivalente a "fond memories". Puede ser algo como: Guardo/Tengo excelentes/caros/hermosos/magníficos recuerdos de ustedes/vosotros.