As caduseus implies - but does not make explicit - you are making mead not brewing beer so what folk do when they brew does not always apply when they ferment honey and not grains. Grains (malt) are very susceptible to bacterial infections in ways that sucrose and fructose is not and honey is essentially made up of the latter two sugars, not complex sugars that need to be broken down by enzymes.
But vinegar does not happen unless several conditions are met. First , the solution is full of alcohol, second, the alcohol is exposed to oxygen and third, the oxygen exposed alcohol is infected by aceto-bacter. Three conditions. But while the yeast is active in your primary it is producing pounds of CO2 and that CO2 is blanketing your mead preventing any O2 from combining with the solution and any aceto-bacter (most often carried by fruit flies) cannot effectively infect your mead WHILE IN THE PRIMARY. Of course, if your meadery is infested with fruit flies you may want to take additional steps to make certain that those flies do not end up (in numbers) in your mead and that may mean that you do need to seal your fermentation vessel...
Once active fermentation has ceased and the yeast are not producing CO2 then that blanket of gas no longer sits above your mead. And that is when you need to have (past tense) racked your mead into a container with a) no head room and b) sealed with an airlock. Banging home a bung and airlock while the yeast is belching out CO2 is like wearing a belt and suspenders to prevent your pants from falling down. Actually, it is more silly as the airlock will prevent more of the CO2 from escaping and that acidifies the mead (stressing the yeast) and stresses the yeast mechanically as the huge volume of gas now super saturating the mead puts great pressure on the yeast cells. An airlock is used in the secondary. In the primary a clean cloth is all you need.