capital one use miles for business class from USA to Vietnam, what are the reasonable estimate in miles needed for a round trip
Booking business class from the USA to Vietnam with Capital One miles is very doable, and the math works out reasonably well if you play it right. Capital One miles are fixed at one cent per mile when used as a statement credit against travel purchases, but the smarter move is to transfer them to airline partners, where you can extract considerably more value.
The most useful transfer partners for this route are Turkish Airlines Miles&Smiles, Air France/KLM Flying Blue, and Singapore Airlines KrisFlyer. Turkish is often the standout here, they have a genuinely excellent award chart for Star Alliance partners, and you can price Vietnam-bound flights on a partner like United or ANA at rates that would be hard to beat elsewhere. Expect to spend somewhere in the range of 70,000 to 90,000 miles one-way in business class from the US to Vietnam on a strong partner program, which puts a round trip at roughly 140,000 to 180,000 miles as a reasonable ballpark. Flying Blue prices dynamically these days, so that number can swing, but starting from around 60,000 miles one-way to Europe and beyond gives you a sense of the floor. Vietnam sits farther, so add accordingly.
If you use Capital One's fixed-value redemption at one cent per mile rather than transferring, you'd need around 200,000 to 300,000 miles to cover a round-trip business class ticket priced at $2,000 to $3,000, which is the typical cash range for decent sale fares. That's a perfectly fine redemption if you find a great cash fare and don't want to deal with award availability headaches, but transferring to a partner and hunting the right availability will almost always beat it on pure value.
The Capital One Venture X is the card to have for this strategy. The 75,000-mile welcome bonus gets you most of the way to a one-way ticket right out of the gate, and the earning rates on travel booked through Capital One are strong enough to keep accumulating efficiently. Given that you're thinking seriously about business class travel now, this card should absolutely be in your wallet.