An overestimate though. That is about the distance following the Earth's surface. But in 3D the distance is smaller.

The 3D distance, piercing through the Earth, is actually about 6973 miles, or 417,876,814 inches.

This is obtained by taking the geo-coordinates of the Pentagon (38.87091,-77.05588) and around the middle of Vietnam (17.5938,106.4014). Take those angles into the trigonometric functions gives us 3D coordinates around the unit sphere for the Pentagon (0.174398369,−0.758777835,0.62756785) and Vietnam (−0.269156822,0.914433935,0.302266744). With Pythagoras, the distance between those is 1,761306232. Multiply this by the mean radius of the Earth (3958.8 miles) for an approximation of the 3D distance: 6973 miles.

417,876,814 inches would give this general $4,178,768,140,000. That's in 4th place in terms of nominal GDP, between Japan and Germany.