You cannot use variable in regex that way. You need to do:
/usr/xpg4/bin/awk -v reg="$var_awk" '$0~reg{ print $0 }'
or simply
/usr/xpg4/bin/awk -v reg="$var_awk" '$0~reg'
- Inside
/ /
your variable reg will be used as a literal word. - Quote your shell variables.