Hi demwalk,
" (Numbers auto-changes the column references to the column descriptions and I haven't found a way to turn off that feature.)"
Numbers > Preferences > General
Uncheck the indicated box.

Your VLOOKUP formula:
The error message "The range Items:Table 1::A2:J39 can't be used as a single value." does not make sense unless you have edited the formula since receiving that error message. With the formula as shown, ignoring the tiny mark left of the insertion point, the error message should be: "The range Items:Table 1::CAL can't be used as a single value."
Here's the syntax for VLOOKUP:
VLOOKUP(search-for, columns-range, return-column, close-match)
search-for, --a 'single value' or reference to a cell (A3) containing the value to search for.
columns-range, --the full range of the lookup table (Table 1::A:J) (preserve both columns if filling the formula right or left)
return-column, --a 'single value' telling the column from which the value is to be returned.
The value must be a number. column 1 is the leftmost column of the lookup table (A).
For "CAL", this number is 3 (if using the table in your original post, where CAL is column C),
or 2 (if using the table in your most recent post, where CAL is column B).
close-match --An optional value. If omitted, or set to close-match, the formula will accept the
'largest value less that or equal to" the search-for value. If set to exact-match, the formula will
accept only an exact match, and will return an error if that cannot be found.
For use in the location shown in your original post, and with the lookup table shown in your original post, your VLOOKUP formula should look like the one shown below the Main table below:

VLOOKUP($A3,Table 1::A:K,3,FALSE)
(FALSE will display as "exact-match" in the formula editor.)
Regards,
Barry