** Shares of Valero Energy VLO.N, the second-largest U.S. refiner by capacity, fall ~1% to $113.80

** Company reports first-quarter loss compared with a year-ago profit, weighed down by lower refining margins and around $1 bln in impairment charge related to its West Coast assets

** VLO says its Q1 refining margin down 29.5% from the year earlier

** Core refining business also sees a downturn in Q1, with operating loss of $530 mln vs profit of $1.7 bln last year

** Excluding asset impairment loss, VLO posts Q1 adjusted profit of 89 cents per share compared with analysts' estimate of 42 cents per share - data compiled by LSEG

** Company posts Q1 net loss attributable to stockholders at $595 mln, or $1.90 per share, compared with last year's profit of $1.2 bln, or $3.75 per share

** Including session's moves, VLO shares down 7.3% YTD

(Reporting by Sumit Saha in Bengaluru)

((Sumit.Saha@thomsonreuters.com;))