specifics in Spear fighting combat

[Robert Vermaat]

Top left is impractical

Yes, when held exactly like on the vase, but see the other comments:

You have the additional option of tucking the spear up under your arm, like a lancer, and jabbing out. (This last position has been advocated here on Rat in the Roman section for late roman spears on a recent thread).

and:

..however, as you are a Late Roman, I am sure I have seen the 'high underhand' depicted by you in photos on other threads, even if it is not too clear from Paul's pot images....it is the one where the spear is tucked relatively high under the armpit, elbow out, whereas the low thrust has the arm almost full length, and the stab comes up from below the waist, with a bend of the elbow/forearm in a fairly vertical arc...

Indeed. My comment was a bit meant to draw out information, but I see it was recognised for what it was.
yes, like Paul said, the 'high underhand' or 'high underarm' grip is advocated among Late Roman re-enactors, following Stephenson I think(Stephenson, Ian P. (1999): Roman Infantry Equipment, the Later Empire, (Tempus)) as a way to extend the lance but still retaining enough to control to jab with it.
The image on the vase looked like the hoplite gripped his spear at the back of the lance, but the weapon was not high enough to compare it to the 'tuck under your arm' position, more like halfway. I'm glad it was recognised as such anyway.