In the BC Vault application, you can also see the current approximate fiat currency (USD/EUR/...) value of all your cryptocurrencies. This is just an informative approximation of current exchange value based on average rates provided by different crypto exchanges as every single one of crypto exchanges has a slightly different exchange rate (as with banks in terms of currency conversion for example).
It has no impact whatsoever on your crypto as all operations on blockchain happen in crypto, not fiat. Blockchain does not care about fiat value at all.
All conversions of crypto to fiat happen on crypto exchanges, where you exchange your crypto for fiat per their actual exchange rate.