And why didn't white play Nf4 sooner? I think 29. Nf4 Qe8 30. Ng6+ looks like winning.

<20...Bxh4> is a mystery to me too.

As far as playing 29.Nf4 Black can play 29...Qg4 rather than your suggestion of 29...Qe8. And after 29...Qg4 play might continue: 30.Kg2 Bxh4 31. Ng6+ Rxg6 32. Qxg6 Qe2+ 33. Kg1 Qe3+ 34. Kh1 Rf2 35. Rxf2 Qxf2 36.Qf5 Bxg3 37. Qxf2 Bxf2 38.Bg6 Kg8 39.Kg2 Bh4 and White has a very good position.

I believe there were two reasons for this move. One point was to divert one of the Black pieces attacking the Pawn on c3. If Black had been able to play ... Rxc3 with attack on the White Queen, it could have granted him some initiative. The other point behind this move can be seen two moves later. After 38. Ne5, White has a strong threat of 39. Rf7+ followed by 39. ... Kh8 40. Ng6+ winning the Black Queen (40. ... Qxg6 41. Bxg6). This threat (39. Rf7+ and following moves) only exists because the 7th rank is blocked by the Black Bishop, which played 36. ... Bxe7 in reply to the move that is the subject of your question. Note that the best Black could come up with at his move 38 was to move this Bishop to unblock the rank. This would not have been necessary if the Bishop had not been diverted from b4 to e7.

As for 29. Nf4, I agree that 29...Qg4 is possible, but the same threats are there. And the doesn't have to go to f4 on move 29; the opportunity was there for several moves. Fritz, channeling through <patzer2> (see his post, above), thinks 31. Nf4 was an improvement over the move actually played.

Of course Black has other defenses but this looks worth investigating.

patzer2: <I can't tell from your analysis why 20...Bxh4 is bad. How did it weaken black or contribute to white's game?> It's not so much that 20...Bxh4 is bad or worse than the declined alternative(s). I think it's just that 20. h4! is strong, driving the Bishop off a strong diagonal and away from any credible defense of the Kingside. For example 20. h4! Bxh4 21. g3 Bg5 22. f4! Bd8 23. Qg6! and Black has a strong advantage with the initiative.

However, as you indicate, declining the pawn offer doesn't seem to help Black's situation any.

On another note, this young Vietnamese GM shows great promise in his understanding of both tactics and positional play. It will be interesting to see how he fares against the super GMs.

al wazir for one thing the move 20...Bxh4 costs two tempi (one to capture the pawn and in the line 20...Bxh4 21.g3 Bd8 22.Qg6 another when White forces back the bishop by 21.g3.Time is one of the key elements of chess, and by spending two tempi,Black is forced on the defensive. Secondly, due to the capture 20...Bxh4 White gets to aggressively post his queen in a dangerous position which wins another tempo by creating a double attack against the weak Black e-pawn.Thirdly, in the line 20...Bxh4 21.g3 Bd8 22.Qg6 Re8 23.f4 Re7 24.f5 exf5 25.Qxf5 White creates the threat of 26.Qf8+ and after Black plays a move to deal with this threat ie. 25...Bc7 White can play 26.Ne2 and after 26...Rd8 continue with 27.Nf4 Rf7 28.Qe6 Re8 29.Qxc6 bxc6 30.Be6 Rxe6 31.Nxe6 Re7 32.Rf8+ Kh7 (the only move) 33.Nxc7 Rxc7 34.Re8 Kg6 35.e6 Nc8 36.Rd8 Nb6 37.Re1 Re7 38.Rd6 Na4 39.Re2 Kf5 40.Rxc6

As subtle as a Sherman tank-but will have to do until something better comes along.