President Reagan vetoed a $13 billion appropriations bill yesterday that includes operating funds for the Treasury Department, the U.S. Postal Service and the White House, and accused Congress of an "ingrained incapacity" to deal with the deficit.

The vetoed measure was $951.1 million more than the administration requested, but a spokesman for House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said that House Appropriations Committee figures showed the bill was $324 million under what the congressional budget resolution ceiling.

"The president's people were looking for something to veto this weekend," said Christoper J. Matthews, the O'Neill spokesman. "This bill was under budget."

It was Reagan's 42nd veto of his presidency and his third this year.

The House approved the appropriation bill 237 to 171 and the Senate cleared the measure by voice vote. Neither vote indicates overwhelming support for the bill, making it uncertain whether Congress could override the veto.

Reagan used the veto to criticize Congress for putting off action on a controversial deficit-reduction amendment to the debt-ceiling extension.

"The solution to solving the deficit problem is not to be found in spending more money," he said. "The need to veto this unacceptable measure is proof positive of the need to sign acceptable legislation to improve the budget process, such as the amendment proposed by Sens. [Phil] Gramm, [Warren B.] Rudman and [Ernest F.] Hollings."

In other action yesterday, Reagan signed into law a 28-day extension of current milk price support levels, heading off what otherwise would have been a $2 million-a-day windfall for dairy farmers.

Included in the measure were provisions that had been added by tobacco-state senators, cutting the price-support level for burley tobacco and slashing the assessment on growers that is used to help pay for the support program.

The bill extends current dairy price supports through Dec. 13, as well as extending certain food-stamp pilot programs and Puerto Rico's cash food aid system. It also gives the agriculture secretary authority to delay scheduled farmer referenda on cotton and peanut marketing quotas.