I'll answer Duncan first.
It is on ALL NFL SIDES for that week ( Sun and Mon night usually ). You just have to play them fri between 8am and 8pm EST. They get a lot of customers who will then turn around and play other sports, horses, casino games, and NFL totals, NCAA games, and NFL games on Sunday after spread moves etc. Many bettors don't like to play that early, they wait to see what the experts say and what late injury and weather reports indicate. If you don't worry about such things, and stick to NFL sides mostly there, it is a fabulous deal. The hoops are for Sat games only typically.
I don't play teasers at all. I believe the way to calculate their "vig" merit would be to add up each half pt worth from the starting number for all 12 halfs. I've not done the math, I may if I get bored one day, but my guess is that you'd have to cross the 3, and probably the 7, to make it worthwhile mathematically based on historical lines. So I would guess +1.5 to +7.5, and +2 to +8 are worth it, as would -8 to -2 and -7.5 to -1.5. I don't know - just guessing. 7 pt teasers for -3.5 to +3.5 would also maybe qualify. If a real expert knows, I'd like to see the math myself.
Parlays I can comment on, I am a self proclaimed "expert" on them!!
ZigZag is correct that 13-5 two teamers have "slightly" more vig than straight bets. 13.5-5 have LESS VIG. Outs like AllWorld that offer 14.5-5 ( 2.9-1 ) are roughly akin to a -103 straight bet. Compared to -110 it is worth considering!
The MAIN reason MOST BETTORS should avoid parlays like the plague is that they COMPOUND losses. If you can't hit >53% of you plays straight, then you lose more betting parlays. Most don't have the skill or $ management discipline to win on straights, so they should never try parlays!
For those who have the $ management discipline under control, and consistently hit >53% long term, there is NOTHING WRONG with two team parlays, IMO. Particularly at 2.9-1. Parlays COMPOUND WINNINGS, just like they compound losses. For the same amount risked, your ROI is more for parlays than straights if you consistently hit above 53% ( not every week but over the long haul ). For three teamers, it is even more advantageous as the compounding is stronger. Your bankroll management must be that much more controlled for 3 teamers though ( as losing streaks are far longer ), but even at 6-1 the VIG is less than straights and the compounding effect is greater than 2 teamers. At INFINITY, I'm getting 7-1 on three teamers ( same as AcesGold no vig parlays on friday ). That's no VIG! With proper $ management, >50% picks will provide winnings long term! Yes, you can go 2-1, or 1-1, on parlays a lot. But you can go 3-0 on one and 0-3 on another and kick ass, or 0-2 and 2-0, having just hit 50%. Over the long run, a >53% capper would win more with parlays ( short term is much choppier though which may be enough to make you avoid them ). Other limitations, like line shopping, are also legitimate reasons to avoid parlays altogether.
So parlays are bad/sucker bets for bad/sucker players who chase and use them to try to get healthy, etc. But if you have a "parlay strategy" with $ management and discipline and hit > 53% they can be a BIG ADVANTAGE. With some of the parlay deals out there you can even win $ with less than 52.38% on them ( like you can with Aces Gold +100 and some of the -105 outs on straights ). They aren't bad or stupid INHERENTLY, just when in the hands of bad or stupid cappers and bettors who don't understand their strengths and weaknesses, and exploit the strengths while avoiding the weaknesses. If you can get a better line elsewhere on two games, then play them straight, but if you can get them both the same at AcesGold on fri, or AllWorld on Sun, there is nothing stupid about playing a parlay on them. Greater than 3 team "spread" parlays are VERY BAD in most every way imaginable and should be avoided other than for "fun and amusement only" for small stakes! If anyone is interested, I posted a chart illustrating the comparative ROI math for parlays some time ago. Do a search here and you'd find it - very revealing, IMHO.
GL