Thankfully I had some very useful comments from Dave Hollins, author of more than a few books about the Austrian army of this period. Not only have I had the honour of getting advice from a renowned expert, but Dave has kindly given permission to post the information he has passed on.
In my research I have managed to get a couple of things confused that Dave has clarified...
Vincenz was the son of GM Martin Knezevich Freiherr von Sankt-Helena and borna s you say in Gracac in the Licca District of the Frontier although the family came from Sankt Helena in Hungary (not Napoleon's last home!) in 1755.
Aged 17, he joined the Hungarian Leibgarde (the ceremonial guard), but in 1775, he became an Unterlt in IR56 before transferring to the Wurmser Hussars as an Oberlt in 1778, rising to Rittmeister and a squadron commander. During the last Turkish War (1788-91) he was a Major in the Vukassovich Freikorps hussars and became their commander, before returning to the Wurmser Hussars as a Major and then joining the Vecsey Hussars in 1792. Promoted to Oberstlt in 1796, he then became an Oberst in 1797 commanding the Erz Josef hussars and licking them into shape, so he and they distinguished themselves at the Adda in 1799, for which Knezevich received the MTO (Militär-Maria-Theresien-Orden) on the battlefield from Suvarov. He further distinguished himself at the Trebbia and Novi as well as the siege of Cuneo.
He was promoted to GM in 1800 and then commanded a brigade based in Styria until 1809. He was appointed FML that year and collected the wreckage of Stoikevich's Korps, which he then led back to Dalmatia and besieged Agram (Zagreb). In that year, he was also made Inhaber of 3. Dragoner and retired in 1812. Appted Vice-Captain of the Kingdom of Croatia/Dalmatia in 1813, he returned to service that year and in 1815 was Military Kommandant of Venetia and promoted to GdK (in the red uniform!) before retiring to his estate in Sankt Helena, where he died aged 77. (Wurzbach, drawing on Hirtenfeld)
Vincenz Knesevich is shown in the painting at his most senior rank of General der Kavallerie, which was the equivalent of the Feldzeugmeister for Generals of Hungarian background, who had commanded or were Inhabers of Hussar regiments. He is wearing the ceremonial dress style of red Hussar uniform for that rank, which was first regulated in 1798 (before then, these Generals could wear a white or red version). He was promoted to Generalmajor in 1800, which was his rank at Marengo.
As such, the figure is actually wearing the Oberrock, worn on campaign, which was a mid-blue at that time with red turnbacks and the single gold lace for GMs. The shoulder sash is worn over the right shoulder, not the left! It is the MTO sash, which was not worn on campaign and has been copied from a formal portrait, and the hat did not have gold edging on campaign - it is rather like Wellington's in the Sharpe progs! The shabraque is fine too as the Generals used a Staff (ie: senior regimental officer) officer's schabraque on campaign.
Many thanks to Dave Hollins for the above information. I'm away to repaint, again, my GM Knesevich and to do a bit of research into GM August Graf Briey de Landres.