Probably a silly question, but why are there no nasal vowels in IPA charts? Should we assume that nasal vowels are placed in the same position as the corresponding oral vowels in the vocal chart? So for example /ɛ̃/ is where /ɛ/ is?
/ɛ̃/ is where /ɛ/ is?
Generic IPA charts don't show you where phonemes are; they show you where phones are. The standard convention would be that the phone [ɛ̃] is in the same "place" as the phone [ɛ]: an open-mid front unrounded vowel. (I don't know enough about phonetics to say whether nasalization causes any predictable distortions to the first and second formants of a vowel.)
That does not mean that the phonetic realization of the phoneme transcribed /ɛ̃/ and the phonetic realization of the phoneme transcribed /ɛ/ are in the same "place" for any particular language. French, one of the most well-known languages with nasal vowels, does not have a straightforward equivalence between the qualities of the nasal vowel phonemes and the qualities of the oral vowel phonemes that are transcribed with the "same" letters. In a typical Parisian accent, /ɛ̃/ is often opener than /ɛ/ (I've read suggestions that an accurate phonetic transcription would be [æ̃]). The French nasal vowel that is typically transcribed as /ɔ̃/ can reportedly have a phonetic value more like [õ] for many speakers in France, and the nasal vowel that is typically transcribed as /ɑ̃/ may apparently be realized with some degree of rounding, so something like [ɒ̃] (or perhaps even raised to [ɔ̃]).
On the other hand, Wikipedia says
In Quebec French, two of the vowels shift in a different direction: /ɔ̃/ → [õ], more or less as in Europe, but /ɛ̃/ → [ẽ] and /ɑ̃/ → [ã].
Good catch. Since its inception, one of the IPA's core principles has been that each phoneme in languages around the world should be given a separate IPA letter. The first of the original 1888 principles stated:
There should be a separate sign for each distinctive sound; that is, for each sound which, being used instead of another, in the same language, can change the meaning of a word.
As you pointed out, this would mean that nasal vowels, aspirated plosives, affricates etc. must also be assigned separate letters because they certainly are distinctive in many languages. But they have never been given separate letters (save for some obsolete letters for affricates), and to do so would have cluttered the alphabet considerably. Although the 1888 principles stated "The alphabet should consist ... as few new letters as possible being used", this apparent discrepancy was not explicitly addressed by the Association until the 1989 overhaul of the alphabet and its principles. At the Kiel Convention of 1989, the IPA revised the principles, which said in part:
It is not possible to dispense entirely with diacritics. The International Phonetic Association recommends that their use be limited as far as possible to the following cases:
(i) For denoting length, stress and pitch.
(ii) For representing minute shades of sounds.
(iii) When the introduction of a single diacritic obviates the necessity for designing a number of new symbols (as, for instance, in the representation of nasalized vowels).
So there. Creating a separate symbol for each phoneme ever found in any language of the world would very soon render the IPA chart pretty unwieldy. So in general, the IPA has, since the 1979 revision, opted for a more simple and efficient set of symbols and diacritics, disposing of superfluous and/or seldom-used letters and diacritics like [ƾ] ( = [ts]) and [ɼ] ( = [r̝]).