If you have a smart phone, just download the Google Translate app (or use the webapp). Don't need to buy a separate device (and I haven't see one, Google included, that can pronounce anything well). It will be good for signs, menus, and other written items.

For data, you can pick up a cheap SIM card for your unlocked device... $2US/mo for unlimited 3G data (throttled after 5Gb), and includes a good block of minutes for phone calls.

If you're sticking around District 1 (Quan 1), you probably won't need it anyway. Enough English to get you by.

One word of caution... make sure you're in a secure, out of the way place before you use your device. Drive-by snatchings on scooters happen. Don't make yourself an easy target... just back yourself up into a doorway or something.