When the vapor pressure is equal to the external pressure, there will form a bubble.
Not true. Instead, when the vapor pressure is equal to the external pressure, then any existing bubbles will begin growing continuously.
And, if no bubbles are already present, then the water will superheat far above the boiling temperature, yet no bubbles will appear. For vapor pressure to exist within water, first gas pockets must exist within water. We need gas-fluid interfaces. Without these, all boiling takes place silently, at the surface where water touches air.
Beware of common misconceptions. Boiling-bubbles are typically not seeded by dirt or contamination. ("Mythbusters" show got it wrong![1]) Instead, boiling is seeded by existing micro-bubbles trapped in small crevices. So, yes, your coffee heated in a microwave oven can superheat and explode, even though coffee is very impure water. If the surface of your ceramic mug lacks air-filled micro-scratches, the coffee will not boil until its temperature is raised far above 100C. But liquids may refuse to boil even when up against a very rough surface, if that surface has been previously wetted with water over 100C. The hot water fills the microscopic roughness with steam, which then condenses, removing any air pockets that let the rough surface act like a "seed" for roiling boil. A fully-wetted rough surface won't prevent superheating.
On a typical stove with a metal pot, the metal bottom will be heated far above 100C, even though the water has not yet approached 100C. The metal surface will be covered with spontaneous steam pockets, but these bubbles cannot grow, since they're right against cooler, under-100C water. When the main volume of water reaches 100C, seed-bubbles are already present on the hot metal, so the pot will immediately begin a visible boil. But without this large temperature excursion at the metal surface, visible boiling may not commence.
Often with new glass cookware, (with no scratches,) and with water heated on a gas stove (with no tiny hot-spots,) the water won't boil. Instead it superheats far above 100C, then unexpectedly produces a few spontaneous micro-bubbles, and exhibits the boiling-explosions called "bumping." The explosions may splash boiling water out of the container. Sometimes they're violent enough to shatter glass. To prevent this, use 'boiling stones' sold in laboratory supply catalogs. Or with microwaved coffee, provide a dry, air-filled wooden stir-stick. These cause a roiling boil at 100C, which cools the bulk liquid and halts superheating.
The 'boiling stones' commonly used to prevent bumping-explosions in laboratory glassware won't work if cooled and then quickly re-used. This happens because all their internal crevices become filled with water. No small bubbles, no "boiling seeds." Restore your 'boiling stones' before re-use, by allowing them to dry thoroughly so the air again fills all of their internal pockets. Or, use a 150C drying oven to rapidly boil off their trapped water. Similarly, a sodden, well-boiled wooden stick in your microwave coffee may stop working. So turn it over and use the dry end!
[1]Big caveat: if micro-bubbles aren't present anywhere, then superheated water will continue to rise in temperature, finally boiling spontaneously with explosive violence. The "seed" or "trigger" for this boiling can be: intrusion of a dry object, vibration and sound waves, thermal fluctuations, ionization from background radiation, and yes, suspended particles, surface roughness, and contamination. Mythbusters(tm) did have it right, but only regarding extremely superheated water. When we first eliminate all surface micro-bubbles and then raise the temperature far past boiling, eventually something will trigger a steam-explosion. The center of this explosive vapor-production might be a tiny grain of dirt, or a pencil tapping against the container, or even a cosmic-ray strike.
