They have many holes in the armor kind of, were they effective against arrows and such?

Barely, if at all.

Breastplates such as the ones depicted in Dances with Wolves have been thought of as perhaps an evolutionary remnant of torso armor worn by Plains Indians such as in "Native North American Armor, Shields, and Fortifications" by David E. Jones*.

Another item of "traditional" clothing among historic Plains Indians, the so-called "hair-pipe breastplate," appeared to derive its form from early types of armor - a natural conclusion particularly among those familiar with examples of American Indian rod-armor widespread both in the Northeast Culture Area and west of the Rocky Mountains.

However Jones also refers to the idea as "basically ill-informed" and notes that the introduction of hair-pipes is from White traders and manufacturers, with the invention of the hair-pipe breastplate attributed to the Comanche around 1854 (meaning that it'd be a fairly new addition to the Plains Indian wardrobe by the time the film takes place).

Though the hair-pipe breastplate itself isn't necessarily a descendant of earlier forms of breastplates worn by Pre-contact Plains Indians, their presence is consistent with how with the introduction of gunpowder to the Plains, armor began to take a more ceremonial and religious function as opposed to utilitarian.

This is easiest to notice with the retained presence of shields long past their obsolescence (with some exceptions) with the transition from warfare being waged either entirely or largely with arrows to the widespread adoption of firearms. This is also present in other forms of regalia such as the headdresses that many associate with the Plains Indian, as war-bonnets and buffalo caps have their origins in armor worn by Indians prior to the introduction of the horse and gunpowder.

These earlier forms of war shirts (often made of a few layers of heavy hide), war-bonnets, buffalo caps, and shields would actually be intended for use against projectiles, blunt and stabbing weapons. Though as previously stated, by the Indian Wars these gradually evolved into the forms that are commonly seen in Westerns, Powwows, etc and no longer constructed with the intent of absorbing arrows or spear blows.

*I would highly recommend for any interested in early/pre-contact warfare across North America to check it out. I specialize in warfare across the Pacific Northwest and found this to be a very convenient resource.