VAT-79 is not to be confused with VAT-69. It's not a whiskey, but an acronym for a type of national sales tax known as the "Value-Added Tax." And fiscal 1979 could see its belated emergence, thanks to the growing revolt against higher Social Security payroll levies.

The relatively painless VAT formula, already so successfully established in most of Western Europe, has long intrigued fiscal experts in Congress, but the idea has been more or less dormant since President Nixon almost embraced it in 1972.

Now, out of the blue, there is a stirring of new interest because Congress, stampeding toward repeal of the sharply increased Social Security payroll deductions, is searching for a substitute way of raising revenue.

Although there appears to be overwhelming sentiment in both chambers for rolling back the recent Social Security changes, there is nothing but disagreement on how to find replacement funds if the repeal goes through.

When the House Democratic Caucus voted (150 to 57) earlier this month for, cuts in the Social Security taxes, if also called for the use of general funds from the Treasury to pay part of the rising costs of the retirement benefits. Originally, President Carter called for the same kind of funding, but that was last year. Now he rightly fears it would reduce or eliminate the income tax cuts he has in mind this year.

More to the point, though, is that the two most influential tax legislators in Congress also are cool to partially financing the retirement system through general revenues. The same legislators, however, have a friendly interst in VAT.

The gentlemen in question are Sen. Russell Long (D-La.), chairman of the Senate Finance Committee, and Rep. Al Ullman (D-Ore.), chairman of the House Ways and Means Committee. Since all tax legislation is under the jurisdiction of these two committees, it is hard to overestimate the power of Long and Ullman in fiscal matters.

The Louisiana senator has all along opposed tapping general revenues to rebuild Social Security reserves. During hearings last year, Long called on Bert Lance, then the administration's budget director, to "consider substituting a value-added tax for some of the needed Social Security funds."

Ullman also said he considered VAT "a very promising option" as an alternative source of revenue for Social Security. He thinks an American version of VAT - invented by the French - should exempt food, medicines and other essentials, but that would pose no serious problem.

Nixon, after almost adopting VAT, finally shied away because critics tagged it as a "regressive" tax that was not based on ability to pay, as supposedly with the income tax. Since then the continuing exposure of tax shelters and other abuses, often resulting in little or no payments by wealthy individuals and profitable corporations, has put the income tax in a less respected light.

Moreover, it now can hardly be argued that VAT is more regressive than Social Security payroll deductions, state sales taxes, and property and education taxes, which are such a heavy burden for low-income families. VAT could be used to cushion all of these onerous levies. It could even be used to lower the corporation income tax.

Although it sounds complicated, VAT is really quite simple in concept and administration. It is a general tax on sales of all commodities and commercial services. It is imposed at each stage of production, from raw materials to retail sales.

Experience in Europe has shown it to be remarkably effectice, since, unlike the income tax, it is virtually impervious to cheating or circumvention. In other countries the value-added tax rate runs from about 10 to 20 percent. In the United States, that would mean $60 billion to $120 billion a year.

Another virtue is that it would help American foreign trade, which is currently deeply in the red. In many European countries, VAT is imposed on imports, making them most costly, but is refunded on goods or export, making them less expensive and hence more competitive.

As Prof. Dan Throop Smith says, "no tax is a good tax in an absolute sense." The case for VAT, he adds, "is simply that it is the least bad tax available," especially if we need substantial additional revenue, but can't get it because of resistance to higher income taxes for individuals and corporations.