[Solved] Typesetting a single word with TeX Gyre Pagella at T5, using Palatino for everything else

Cactus Asks: Typesetting a single word with TeX Gyre Pagella at T5, using Palatino for everything else
I need to typeset a single Vietnamese word in my otherwise English document. My document is in Palatino, which doesn’t support T5, so after I switch to T5, it uses a different font. So far, this is the same problem as described in this question. However, there the solution was to change the main font to TeX Gyre Pagella. I’d like to apply that solution locally, i.e. use TeX Gyre Pagella just for this one word.

Code:
documentclass{memoir}
usepackage[T1,T5]{fontenc}
usepackage{fontspec}
usepackage[defaultsups]{newpxtext}
usepackage[english,vietnamese]{babel}
newfontfamilyvnfont{TeX Gyre Pagella}

begin{document}
Nguyen

noindent

{fontencoding{T5}vnfontselectfont{}Nguy~ecircumflex{}n}
end{document}

However, this fails with:

LaTeX Error: Command ecircumflex unavailable in encoding TU.

If instead I write vnfontfontencoding{T5}, then the latter overrides the former and I get the non-Palatino-looking font.

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