The use of "to riot in understatement"

Solution 1:

Read to riot as to revel, to indulge, or something similar, and you'll get the gist: it's a writerly aside, expressing how he is reveling in understatement.

Here is the Oxford English Dictionary, "riot, v.," I.3.a, with the latest example:

a. To take great delight or pleasure in something; to revel in. Now rare.

1920 South Atlantic Q. Apr. 170 Ward rioted in misery and comforted himself with reflections on the general stupidity and perverse worthlessness of by far the major part of mankind.

To riot in has a sense of taking pleasure irrespective of one's own better interest. Hence Merriam-Webster adds a sense of wantonness (verb, def. 2):

2 : to indulge in revelry or wantonness

So when Vidal employs this as his own turn of phrase, he suggests both that he is engaging in understatement and that he is playing. So it's somewhat like "to say the least," but more verbose, literary, and even wanton.