No doubt muriatic acid will do the job at 5 to 1 (5 parts water to 1 part muriatic), but 10 to 1 would be safer. 10 to 1 is the highest concentration recommended in the ceramic tile trade for acid washing glazed tiles, because higher concentrations can etch the glaze which enables crud to stick better. The porcelain bowl is made much like a porcelain tile, just different in thickness. Brand name vinegar is usually at a 5% strength, which would be about 10 to 1 for muriatic, but even at 5% muriatic is more tenacious than vinegar.
Concentration is only part of acid strength. Vinegar is acetic acid, a fairly weak acid. Muriatic acid is hydrochloric acid, a MUCH stronger acid. Even glacial acetic acid (essentially, pure acetic acid) is nowhere near as nasty as 10% HCl(aq) (a pretty strong solution of hydrochloric acid).
The strongest hydrochloric acid is about 38% HCl(aq). "Muriatic acid" is usually marketed at about 31 or 32% HCl(aq). Even diluted 10:1 (or ~3% HCl(aq)) your muriatic acid will have a pH of about 1.5, while glacial acetic acid will have a pH of about 2.4 (remember, pH is a log scale and lower values are more acidic). In other words a 10:1 dilution of muriatic acid will be MUCH more acidic than pure acetic acid.
ALSO, never, never, NEVER pour water into a strong acid (like muriatic acid).
When diluting an acid, always, always, ALWAYS pour a strong acid into water. When water and a strong acid mix it generates heat, quite a bit of heat depending upon the type of acid and its concentration. If you pour the acid into the water, the water will act as a heat sink, and its heat capacity will prevent things from getting too hot. If you pour the water into the acid, there won't be enough water (initially) to absorb the heat generated and you could cause the water to boil, essentially causing a steam explosion, which would toss hot acid all over you and everything/everyone else nearby. I have seen the results of just such a mistake, and believe me they are NOT pretty.
Never forget them. Do something to prevent it from happening again.
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