In this case the c is just a c (pr. tz), not a č or ć (pr. ch). Although in some languages the 'vece' part turned into 'veće' (like in modern Croatian) and we have two versions of the name (Vjeceslav and Vjećeslav), it is a normal c in Vaclac, Veceslavb, etc.
About spelling Veceslavb in Cyrillic it depends on the language, I only know Serbian Cyrillic and it would be 'Вeцecлaвб'.
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