No, it doesn't matter whether the following word starts with a vowel or a consonant sound.
However, there are three different indefinite articles in Norwegian, which correspond to the grammatical gender of the noun they modify:
en = indefinite article for masculine nouns.
ei = indefinite article for feminine* nouns.
et = indefinite article for neuter nouns.
*feminine nouns may be declined as if they were masculine, so they can take "en" in place of "ei".
Note that grammatical gender does not correspond with biological gender.
In English, bread and water are uncountable nouns which means they cannot be counted ("one bread", "two breads"; "one water", "two waters") or take the indefinite article "a/an". However, you can say "a loaf of bread", "two loaves of bread" as well as "one glass of water" or "two bottles of water".
Both brød and vann are nouns of neuter gender. Unlike in English, brød is countable in Norwegian so et brød translates to "one loaf of bread". In this sentence, it isn't explicitly stated so the correct translation is just "bread" because "a bread" is grammatically incorrect.
On the other hand, vann is uncountable in both languages. However, et vann has a translation - "a lake". Below is an example of how one can use this uncountable noun to indicate exact measurement.
Du har et glass vann. - You have a glass of water.
Sometimes, uncountable nouns can be expressed by using the plural form but this is meant as different kinds of something, and not as "many, two or five" instances of that noun.
Hun har mange oljer. - She has many oils. (bergamot, eucalyptus, ylang-ylang)
Click here for more information about uncountable nouns.