They do not. For example, Russians devoice final consonants, while Serbs do not. There is vowel reduction in Russian but there isn't in Serbian. Serbs do not palatalize consonants before [i].You can't, really: All that we can hear is that it is a Slavic accent: Russian and Serbian accent sounds absolutely the same in English.
Do you mean they are pronounced like diphtongs? Or what? Could you give some examples?Some Russian vowels that are a bit like a mix of two different vowels (from my perspective).