Learn Portuguese (Portugal): Vou comprar este vestido hoje.

Este translates to "this" and is used for objects physically close to the speaker (like a dress they are currently holding). If the dress were closer to the person listening, you would use esse. It is masculine to match the noun vestido, which is why it is not the feminine esta.

Is vestido really a masculine word even though it means "dress"?

Yes! Grammatical gender does not always match who wears the item in real life. Because vestido ends in -o, it follows the standard rule and is a masculine noun. This is exactly why we must pair it with the masculine determiner este.

How does vou comprar translate to "I am going to buy"?

This uses a very common shortcut for the future tense. It combines the present tense of ir (vou = I go / I am going) with the infinitive, or dictionary form, of the main verb comprar (to buy). You only have to conjugate the first verb!

Where is the word eu (I) in this sentence?

In Portuguese, it is perfectly normal to drop the subject pronoun. The verb vou is only ever used for eu, so the listener already knows exactly who is doing the buying. You can absolutely say Eu vou comprar, but leaving eu out sounds much more natural.