Cows and milk

Does anybody know which PIE root the Latin noun vacca (cow) came from?

Besides, there's question that I find interesting. Why most linguists
don't seem to agree in the identification of a single PIE noun
for 'milk'? I've read about Sanskrit "dugdha" (milk), apparently
derived from a root duh-. Is that right? This word indeed doesn't seem
to be related at all with the root *melg- / *mlg- (Latin mulgere 'to
milk', Proto-Slavic melko 'milk', Proto-Germanic meluk 'milk', Albanian
mjelë 'to milk', etc.) However, why another root *glagt- has been
proposed to explain Latin lac, lactis 'milk' and Greek gala,
galaktos 'milk'? Am I the only one to whom all these forms (sharing the
internal sequence -lg-) look somewhat similar?

Thanks