If this is B737NG specific, then remember your aircraft is happy flying in the low drag bucket of 40-260KIAS. If you were in ECON, at F370, depepnding on weight you'd have been at M0.77? Come down to F350 and the ECON would be about M0.755 or similar. Neither is anywhere near limiting and to be honest if you left it in MACH then the resulting speed at F350 is still less than LRC, the top end of that efficient low drag range of KIAS.
Me? I'd just ALT Intervene and let the VNAV deal with it. It will select the ECON cruise speed and adjust to it on the way down. The biggest problem with V/S is generally the lack of auto changeover. Put simply, in MACH, your KIAS goes up in descent, in IAS, the MACH decreases. But then you should know that.