The procedure is simple. The health care provider simply swabs vinegar, i.e. acetic acid, on the cervix and looks for areas that change color. Normal cervical tissue remains unaffected by the acetic acid, but damaged tissue -- such as that found in pre-cancerous or cancerous lesions -- turns white. The provider can then remove the damaged tissue on the spot using cryotherapy or other techniques, or they can perform a biopsy for further follow-up.
Most studies have found that VIA, and its cousin VLI -- visual inspection with Lugol's iodine -- are somewhat less specific than Pap smears, but more sensitive. In plain terms, that means they generally detect more early lesions but are also more prone to false positives. This could conceivably lead to overtreatment, but in low-resource areas where large numbers of women are still dying of cervical cancer, some governments have decided it is a worthwhile trade-off.
Overall, VIA seems to be an excellent cervical cancer screening method for use in low-resource settings where Pap smears and HPV tests are inappropriate due to either lack of expertise or high per-test cost. The general consensus is that VIA is just as useful as the Pap smear; it's just a matter of determining which one is more appropriate in any given circumstance based on availability of trained personnel for screening and followup.
It is also worth noting that VIA has an advantage for traditionally under-served patients who may have difficulty coming in for follow-up care. Since screening and treatment are done at the same visit, there is no need to wait for results and then return to the doctor. There are many people in the world for whom getting to the doctor requires enormous amounts of time, expense, and negotiation, and for them being able to deal with screening and treatment in a single visit can greatly reduce the personal and financial burdens of health care.
Chumworathayi B, Limpaphayom K, Srisupundit S, Lumbiganon P. VIA and cryotherapy: doing what's best. J Med Assoc Thai. 2006 Aug;89(8):1333-9.
Gaffikin L, Lauterbach M, Blumenthal PD. Performance of visual inspection with acetic acid for cervical cancer screening: a qualitative summary of evidence to date. Obstet Gynecol Surv. 2003 Aug;58(8):543-50
Qureshi S, Das V, Zahra F. Evaluation of visual inspection with acetic acid and Lugol's iodine as cervical cancer screening tools in a low-resource setting. Trop Doct. 2010 Jan;40(1):9-12.