How can I-526 derivatives adjust status when their status is different from the petitioner?

You can still marry her after your I-526 is approved. Then she can come here and adjust her status here with you.

Once you marry your girlfriend before you file your I-485, she can come to the U.S. with an F-2 visa. Then, after your I-526 is approved, both of you can file an I-485 in the U.S., assuming that both of you are still in valid non-immigrant status.

If the investor files an I-526 petition in the U.S. and is able to file for adjustment to conditional permanent residency in the U.S., then you as the spouse, if you are maintaining legal F-1 student status may also file for adjustment in the U.S.

In order to file an I-485, an applicant must be in valid, nonimmigrant status in the United States, and must be otherwise eligible to adjust status. If the applicant is not in the United States or is not in a valid nonimmigrant status, then he or she must apply for an immigrant visa at the U.S. embassy or consulate in their home country. The principal applicant, the EB-5 investor, must obtain lawful permanent residence at the same time or before any dependents. If the dependents will adjust status in the U.S. and the principal applicant will use the immigrant visa process abroad, then the principal applicant must obtain the immigrant visa and enter the U.S. before the dependent files the I-485 application.

The National Visa Center takes each person listed on the I-526 and treats them somewhat individually. Family members can go through the same processes or different processes. Adjusting requires the actual person adjusting to have valid U.S. status.

You do not need to be married until after the I-526 is approved so get married when you are ready. As long as you are married and both in an legal nonimmigrant status then you can both adjust here in the U.S. Technically, your spouse may even follow-to-join you if you marry any time before your two-year conditional green card is issued to you. Under that scenario you would adjust upon I-526 approval and your spouse files an adjustment after you are married (but before your green card is granted.)

While you could file the I-485 adjustment after getting your I-526 approved, you may also opt to go through the DS-260, immigrant visa application so that you and your soon to be spouse could get the immigrant visa at the same time. The only way that your future spouse could file the I-485 with you is if she is already in the U.S. with a valid visa when your I-526 is approved and you are ready to file the adjustment application.

You could both opt for consular processing, but from the facts you state it does not appear that she is eligible to adjust status unless she is here as the Spouse of an F-1 Visa holder at the time of I-526 approval.

You do not say what her status is or even where she is. If she is in Vietnam now, perhaps she could get an F-2 visa in Vietnam after you marry, then she could come to the U.S. and file an I-485 when you do. If she in the U.S. now, it depends on what her status is. If she is in a lawful nonimmigrant status, then she too could file an I-485.