The = field doesn't let you use 'any old VBA function'. It only has arithmetic/logical operators and functions (such as MAX, MIN, SUM, PRODUCT etc.) It does not have any text or date/time functions or operators.
You can find a list of the available functions and operators here .
[Except that there are one or two things missing from that description - in Word, the = field does also have an IF function ( { =IF(a,b,c) }, although it is missing from the ECMA and ISO standards, and the description of table references is incomplete].
Unfortunately, there is no other field type in Word that will reliably do what you want. You can "nearly" use a DATABASE field to do it, but that is complicated, will introduce a paragraph mark that you cannot get rid of in modern versions of Word, will not work on Mac word (or online versions, AFAIK), and will fail if for example the text you are LEFTing contains a " (or perhaps ') character.
{ =LEFT("{ REF _Ref436832789 }", 10) }
, which produces the expected result for the nested field, but produces an error when the formula is toggled to view the result.