Try Liat Air
http://www.liatairline.com/

They seem to have regular flights

Carlos: actually SVG does not operate any single-engine planes - they are all twin-engine (AC 500's, C402's and BN2 Islanders). The aforementioned are twin-engine piston aircraft, whilst the Twin Otters that they operate on longer routes are twin-engined turbine aircraft.

Mike: Most of the current LIAT flights from St Lucia to St Vincent go via Barbados, meaning that what should be a 35-minute journey ends up taking 5 hours. There are a bunch of new flights listed on the LIAT web site but you will see "no seats available" written against many of these- that's because these are new flights to be operated by Carib Aviation under a code-sharing arrangement with LIAT, but the flights have not yet been inaugurated.

However, if you are able to get booked on the one flight that does go directly, then it will certainly be cheaper than chartering a plane.