Conditional in the past -- "If Vietnam *would* fall..."

President Eisenhower had a strategic vision, the Domino Theory. If Vietnam would fall, other countries in Southeast Asia would fall too.

A lecturer (native speaker) said it on a lesson. Using would confuses me. For me it should be at least "If Vietnam were to fall." And in the orthodox way it should be "If Vietnam had fallen, other countries would have fallen too."

Answer

This question has been under my mind from the start. I know the sentence sounds right, but couldn't explain why. Once I saw Kaz's answer, I read it and I totally agree with the answer, especially the part:

We can understand it as simply conditionally relating two clauses, which are expressed as a past subjunctive form of "to will": a subjunctive, irrealis form of the sentence "If Vietnam will fall, other countries will fall" ...

What I thought was not fully answered is the reason "why" the construct If ... will ..., ... will ... can be used, and "how" one could use it.

Today, I read Swan's Practical English Usage again, and now I think I've found a good answer.

In Swan's book, there are a handful of entries involved,

262 if (7): other structures found in spoken English
262.1 would in both clauses
Conditional would is sometimes used in both clauses of an if-sentence. This is very informal, and is not usually written. It is common in spoken American English.

which is close, but I think is not quite related to this question, except for that How would we feel if this would happen to our family? is acceptable as mentioned.

The treatment of if ... will is explained exclusively under the entry 260. Here is a summary,

260 if (5): if ... will
260.1 results -- We'll go home now if it will make you feel better.

260.2 'if it is true now that ...' -- If prices will really come down in a few months, I'm not going to buy one now.

260.3 indirect questions: I don't know if ... -- I don't know if I'll be ready in time.

260.4 polite requests -- If you will come this way, I'll show you your room.
(Would can be used to make a request even more polite.)

260.5 insistence -- If you will eat so much, it's not surprising you fell ill.

In my opinion, the entry 260.1 answers this question in particular.

260.1 results
We use will with if to talk about what will happen because of possible future actions - to mean 'if this will be the latter result'. Compare:
- I'll give you £100 if I win the lottery. (Winning the lottery is a condition - it must happen first.)

- I'll give you £100 if it'll help you to go on holiday. (The holiday is a result - it follows the gift of money.)

Back to the sentence in questioned: "If Vietnam would fall, other countries in Southeast Asia would fall too." I read Vietnam would fall the first time as a speculation. Reading 260.1, I turned to believe that it is more likely that the lecturer was saying that President Eisenhower said this in a context that Vietnam would fall is a result. I'm not sure which condition it would result from, but since this is about the Domino theory, I believe that it might be a result of some action, or more likely a result from not doing some action.

Here is how I imagine what the lecturer might have wanted to say:

We cannot overlook this matter. If Vietnam would fall (result from overlooking), other countries in Southeast Asia would fall too (result from Vietnam had fallen).