Electrical Adaptor for Brussels, Prague, Budapest and Vienna

No you need the type c and f that will work in Belgium and Hungary Austria and Czechoslovakia
https://www.amazon.com/plug-adapters-country/s?page=1&rh=i%3Aaps%2Ck%3Aplug%20adapters%20by%20country

I would buy one that is convertible

No, a type L (three round pins in a line, click here for picture) is specific to Italy. It will not fit the sockets in any other country.

Is your appliance 2-pin or 3-pin?
If 2-pin, you can get a standard 2-pin Europlug adapter, which would also have worked in Italy (type C, aka "Europlug", click here for picture).
Or, if you have a 3-pin appliance (with earth pin), you need a "Schuko" adapter, compatible with both types E and F (click for picture).

Most of the adapters shown on the Amazon page linked by the first poster are technically not safe in some applications in Europe. They have a hole for the grounding pin on a grounding US plug, but it does not connect to the grounding means in the European receptacle. The first one (a set actually), by Bestek, will work but only one of the eight adapters will be needed where you are going. The one below the Bestek set is the type E/F Schuko to which Chris referred. I like it.

If you are only plugging in USB power supplies, which have two blades and no grounding pin, any of the adapters shown will work, as will the two pin type C adapter. Note, however, that the type C is only rated for 2½ amps (575W at 230V), not enough current for a hair dryer.

Also, any American appliance, like a hair dryer, if it has a polarizing plug (one blade wider than the other), should not be used with any adapter in the countries to which you are going. An appliance with a polarizing plug can be used with a 3-pin grounding, adapter in Switzerland, but in no other country (maybe France, but maybe not).