How to write Spanish Vowel sounds into English?

Background

I'm writing a novel with original character names, and I want to find the way of how to correctly write their names in English to keep the same pronunciation as they had had in Spanish.

The basic problem for me is the sounds of the vowels, so how can you generate the Spanish vowels in English?

Example

Nimree written like this in English would have a different sound than its pronunciation in Spanish, and even tough names writing across languages should be respected. I want to shape these Spanish names into English so there's no confusion about how they are to be pronounced.

So how you would write each original Spanish vowel sound so that it keeps its original sound when ultimately read by English speakers who don't understand Spanish vowels?

• A
• E
• I
• O
• U

“What would Heighssoose do?”

This is not possible, and probably not desirable even if it were. It’s easier to teach people the five vowels and be done with it.

You aren’t going to be able to represent the sounds of Spanish in an unambiguous way that any English speaker will “always” say “right” because the internal phonologies of each language are different, and mutually incompatible.

Plus if you try you’ll get something crazy like ghoti, the would-be “creative” spelling of fish.

• Spanish A > English AH: as in the English word KHAN.
• Spanish E > English EIGH: as in the English word WEIGH except without a Y at the end.
• Spanish I > English EE: as in the English word FLEECE.
• Spanish O > English OAH: as in the English word BOAT except without a W at the end.
• Spanish U > English OO: as in the English word FOOL not like the English word TOOK.

This sort of thing leads to extreme silliness. It renders the names unrecognizable if you try to use English spelling to approximate Spanish sounds. People will hate you if you try this:

• Hwahn for Juan
• Dhahbheethe for David
• Rroahssah for Rosa
• Dhahnyeighll for Daniel
• Ahleighhahndhroah for Alejandro
• Mahnweighll for Manuel
• Lootheeah for Lucía
• Ahntohnyoah for Antonio
• Maheeteigh for Maite
• Pahkoah for Paco
• Frahntheeshkoah for Francisco
• Mahteeahss for Matías
• Seighbhahshtyahnn for Sebastián
• Neekoahlahss for Nicolás
• Hoahsseigh for José
• Beighnhahmeen for Benjamín
• Meighrrtheightheighss for Mercedes
• Klahootheeah for Claudia
• Eethahn for Izan
• Oahleebheeah for Olivia
• Ooghoah for Hugo
• Hoahrrhheigh for Jorge
• Bheetoahrrh for Victor
• Rroobheighn for Rubén
• Eighthwahrthoah for Eduardo
• Eessahbheighlyah for Isabella
• Eezhmaheighll for Ismael
• Kahrroahleenah for Carolina
• Eeneighss for Inés
• Peighthroah for Pedro
• Ahnthreighah for Andrea
• Heighssoose for Jesús

If you attempt this, your readers will not thank you. Just teach them how to say the five simple vowels and be done with it.

Well, and all the consonants, which are also “all” of them different from those of English, as I’ve attempted to represent above.

There are regional differences in vowel pronunciation both in Spanish and English that make this almost impossible.

This is an oversimplification, but here are the five vowel sounds in Spanish:

Here is a chart of "General American" vowels. I'm confused because the chart is labelled pure vowels (monophthongs) but some are clearly diphthongs.

Basically your job is to match the Spanish (a, e, i, o, u) IPA symbols to the words in the General American chart which show the corresponding vowel.

So Spanish 'a' would be the 'a' in "father" or "spa".
Spanish 'e' would be the beginning vowel sound in General American "lake", ie.,/leik/.
Spanish 'i' would be the 'ea' in General American "beam".
Spanish 'o' would be the 'o' in General American "goat".
Spanish 'u' would be the 'oo' in General American "goose".

If you are an American who doesn't speak perfect "General American" as described in this chart (very likely), or you have some idiosyncrasy in some pronunciation (very likely), or are not American at all (likely), or the Spanish is coming from somewhere where the vowel sounds used aren't pure IPA (a, e, i, o, u) then all bets are off and you can't assume anything.

But as a general guide you can try this. Also there is the issue of vowel length which isn't even covered here, but that's the least of your problems.

These charts are screenshots taken from:
Spanish phonology at Wikipedia
General American at Wikipedia