Mr. Vecchione, a Queens native, looks as if he could play a banker in a movie. He sports Hermès ties and collects high-end watches (not including Rolexes, which he says are too common). His pay over the past three years was worth nearly $22 million, including stock.

Until recently, the bank was in a ravenous expansion mode. In 2015, Western Alliance acquired Bridge Bank, a San Francisco lender that competed with Silicon Valley Bank for business from venture capital firms. Like Silicon Valley Bank, Bridge Bank advertised its ability to finance start-ups and other businesses that typically hold more than $250,000 in their bank accounts — a risky proposition, given that the federal government insures deposits only up to that amount, making such accounts flighty.

A so-called commercial lender, Western Alliance mostly lends to businesses, like time-share companies, real estate developers and hoteliers. It has a collection of branches across the West under brands like Bank of Nevada, Torrey Pine Bank and Alliance Bank of Arizona.

As of year end, Western Alliance’s $68 billion in assets made it the 40th-largest lender in the country. The bank’s board of directors had approved a plan to grow as large as $100 billion by expanding outside the West, an initiative that included new Manhattan offices on Madison Avenue whose walls are lined with marble.

Silicon Valley Bank’s demise hit like an “explosion,” said Western Alliance’s chief financial officer, Dale Gibbons. In the hours after it was shuttered, Mr. Gibbons, Mr. Vecchione and their team watched gape-mouthed as their bank’s accounts dwindled. Longstanding clients put in withdrawal requests without so much as a check-in call.