Poland reached tentative agreement yesterday with Western bankers on stretching out its payments to commercial banks this year on the nation's huge foreign debts, according to bankers and officials in Vienna.

A "memorandum of understanding" negotiated in Vienna between the government and six of its leading banks outlines terms of the rescheduling that include new concessions to the almost-bankrupt nation, reports said. Poland--which owes about $25 billion overseas, of which about $11 billion is due to banks--was due to repay $1.5 billion in principal and $1.1 billion of interest to commercial bankers this year. Much of this would be stretched out over several years under the plan.

The leading banks will now report to a working group of 20 banks managing Poland's commercial debt, and they in turn will seek approval for the plan from the 500 or so banks involved. Formal agreement may come in October, bankers said.

Agreement between Poland and Western governments on rescheduling the nation's official debts was delayed by the United States as part of its economic sanctions against the East bloc nation. The sanctions were imposed following the imposition of martial law in Poland in December 1981. However, discussions on these debts are now due to begin again, after the United States last month withdrew its objections to talks on the debts in the wake of the lifting of martial law.

A meeting of Western officials in Paris next month will discuss technical aspects of the stretch-out of Poland's 1983 debts to governments, a Reagan official said yesterday.

That meeting of the "Paris Club" will also consider a rescheduling of the loans made by Western governments to Brazil, an official said. Brazil has now decided to ask governments to reschedule some of its official debts to help ease its liquidity squeeze, the official said.

A communique issued after two days of talks between Polish officials and foreign bankers said "as expected, discussions in Vienna during the last two days between representatives of the Polish bank, Handlowy, and the Finance Ministry, and members of the banks' working party resulted in the signing of a memorandum of understanding dealing with the basis on which maturities falling due in 1983 will be rescheduled, and new trade-related facilities will be made available.

"Discussions on documentation and detailed implementation of the agreement will continue," it said.

Reports out of Vienna said that Poland's bankers have agreed to convert 65 percent of the $1.1 billion of interest due this year into short-term trade credits. The banks want Poland first to pay interest that is due, and then to have this loaned back as trade credits, reports said.

The bankers last year converted some of Poland's interest payments into principal, in what was viewed as a major concession. Last year's agreement provided for only 50 percent of the interest to be converted into new loans, and bankers first offered to convert 60 percent of the 1983 interest this year.

In addition to the consolidation of interest into trade credits, Western banks have tentatively agreed to reschedule 95 percent of the $1.5 billion of principal repayments due this year, making it payable over 10 years, reports from Vienna said. Poland would have to pay a higher rate of interest on this than on its rescheduled 1981 and 1982 debt, reports said. According to these reports, the nation will pay 1 7/8 percent over the London Interbank Offered Rate, compared with 1 3/4 percent.