The Empire of Annam was under French protection since 1886. I've seen an old state flag, that was yellow with two "eastern" (Chinese or Annamite) written-characters in black. After 1886 the yellow flag included the French tricolor in the canton. The national flag was three horizontal strips yellow, red, and yellow. Jaume Ollé, 23 September 1997
A triangular flamuled pennant with the lower edge ortogonal to the hoist, yellow with red orle (inner bordure) with the dragon emblem covering the triangular field. Above the pennant is a yellow and red bicolour long pennant. Željko Heimer, 27 November 2004 Source: Imperial Order of the Dragon of Annam website
A rectangular standard higher than long - an orange field bordered green and red (red outside) and with blue falmmuly border on three outer edges, In the orange fields there is the dragon. Above the flag is an orange long pennant with green and red ornaments. Željko Heimer, 27 November 2004
image by Željko Heimer Proportions: 2:3 Source:Flaggenbuch, 1939 [neu39]
Possible alternate version
image by Jaume Ollé
A yellow flag with the French tricolour in the canton. as with some other French colonial flags of the period, the tricolour height is equal to 1/3 of the hoist.
The 1926 Flaggenbuch[d9e26] shows the same flag captioned National Flag. Jaume gives this flag in square format with square Tricolour 1/2 the hoist high, which might have been a variant. Željko Heimer, 27 November 2004
Flaggenbuch depiction of the "Dragon Flag of Annam"
image by Željko Heimer Proportions: 1:2 Source:Flaggenbuch, 1939 [neu39]
Depiction of the "Dragon flag of Annam"
image by Jaume Ollé
A yellow-red-yellow horizontal trband flag with the central stripe being of the double with then each of the yellow stripes.
Jaume shows it in ratio 2:3, but I guess it may be wandered if the ratio was established at all. Is this flag kind of origin of the latter (South) Vietnamese flag with three red stripes? Željko Heimer, 27 November 2004