Hi,
I understood your question in the way you want to change parts of a text within MTEXT to another font, .... or globally change any formating for parts of your text.
The way AutoCAD does modifying e.g. that the 3rd char within the text to be Arial-Black is to put control-characters within the text. And that is what you have to do: take the text-context in your vb-app, place the control characters into the text and save it back.
So you can learn the special chars AutoCAD uses within a MTEXT you can create a MTEXT-object by just drawing it, modify it in the way you like and look into the property-window how the special-formatting is saved.

Hope that was more clear now (and I understood your question correctly).
- alfred -
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Alfred NESWADBA
ISH-Solutions GmbH / Ingenieur Studio HOLLAUS
www.ish-solutions.at ...
blog.ish-solutions.at ...
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CDay 2026------------------------------------------------------------------------------------
(not an Autodesk consultant)