Is flow attached in center of delta wing or only on the edge where two vortices operate?
Any video of delta wing vs AoA with smoke?
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Yes, flow is attached at the mid section of a delta wing at low angle of attack. This would be the case in the cruise condition of a delta airplane.
At high angle of attack, when the flow separates at the swept leading edge and produces vortex lift, a gusset will sit near the wing surface where the two vortices meet. It consists of very turbulent air which is quite unlike the low-energy deadwater of a normal separation on a straight wing, and the high rate of mixing is also quite the opposite of attached flow. You may call it separated but make sure you do not confuse this with what we normally call separated flow.
It may be a mistake to try to understand slender delta lifting mechanism in straight wing terms. Maybe think of it as 2 wing tips with no wing in between.
Vortexes form at wing tips due to wind shear, just like their full scale tornado cousins do. These vortexes are not attached, yet they are not turbulent either. The flow is organized by the leading edge of the delta shape continually reinforcing the vortex the entire length of the wing, creating lift.
Why not build a straight leading edge with two 1/2 delta wing tips?
This was actually a very popular design over 100 years ago, living on today in the R/C Ugly Stik. Essentially, these were the forerunners of todays winglets, most notably seen in the 1910 Etrich Taube.
As with today's winglets, substantial benefits are had only at higher angles of attack, with wash-out and Hoerner wingtips more commonly seen on modern straight wings.