[viach]

I think if you speak slowly any of the Slavic languages it will be understandable enough "from Prague to Vladivostok"

[vzaliva]

It is a common misconception. Knowing two slavic languages from different branches (e.g. west and east slavic groups) improves your comprehension of other slavic langauges a lot, but even that still often not enough. I speak 2 slavic languages but found myself confused with simple everyday phrases in others.

[Mikeb85]

Yup. Grammar is the same just with some different vocabulary (mainly because of loanwords from other languages that got inserted because of history).