By Davide Barbuscia NEW YORK (Reuters) -Top U.S. asset manager Vanguard is bullish on corporate bonds despite high valuations, and while it expects tariffs will continue to be a risk for the U.S. economic and inflation outlook, those headwinds could be offset by further Federal Reserve interest rate cuts. Investment-grade credit spreads - or the premium over U.S. Treasuries paid by high-rated companies to issue bonds in the U.S. market - declined to 74 basis points last week, their lowest since 1998, a ...